U.S. patent number 7,076,168 [Application Number 09/703,202] was granted by the patent office on 2006-07-11 for method and apparatus for using multicarrier interferometry to enhance optical fiber communications.
This patent grant is currently assigned to Aquity, LLC. Invention is credited to Steve J. Shattil.
United States Patent |
7,076,168 |
Shattil |
July 11, 2006 |
Method and apparatus for using multicarrier interferometry to
enhance optical fiber communications
Abstract
A redundently modulated multicarrier protocol known as Carrier
Interference Multiple Access (CIMA) is used in an optical-fiber
network having wireless links at network nodes. CIMA is a protocol
that can be used to create wireless protocols (such as TDMA and
CDMA) having enhanced capacity and reduced system complexity. A
CIMA optical-fiber network uses dispersion to enhance signal
quality and facilitate switching. CIMA achieves both diversity
benefits and capacity enhancements by providing redundancy in at
least one diversity parameter while providing orthogonality in
another diversity parameter. This basic operating principle of CIMA
may be combined with multi-user detection to achieve frequency
reuse and improved power efficiency. In the wireless link,
diversity may be used to reduce the effects of small-scale fading
on interferometry multiplexing.
Inventors: |
Shattil; Steve J. (Boulder,
CO) |
Assignee: |
Aquity, LLC (San Diego,
CA)
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Family
ID: |
36644164 |
Appl.
No.: |
09/703,202 |
Filed: |
October 31, 2000 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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09022950 |
Feb 12, 1998 |
5955992 |
|
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60163141 |
Nov 2, 1999 |
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Current U.S.
Class: |
398/76; 398/202;
398/78 |
Current CPC
Class: |
H04B
10/25752 (20130101); H04J 14/02 (20130101); H04J
14/0224 (20130101); H04L 5/023 (20130101); H04L
25/03343 (20130101); H04L 27/2647 (20130101) |
Current International
Class: |
H04J
14/00 (20060101) |
Field of
Search: |
;385/29
;398/79,68,152,82,139,76,77,78,115,89,99,202,211 ;375/144
;370/382 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
Linnartz, "Synchronous MC-CDMA in Dispersive Mobile Rayleigh
Channels," Proc. 2.sup.nd IEEE Benelux Sig. Proc. Symposium,
Hilvarenbeek, Mar. 23 2000. cited by other .
Yee, "Controlled Equalization of Multi-Carrier CDMA in an Indoor
Rician Fading Channel," IEICE Trans. on Comm., Japan, vol. E77-B,
No. 7, Jul. 1994. cited by other .
Yee, "Wiener Filtering of Multi-Carrier CDMA in a Rayleigh Fading
Channel," IEEE/ICCC PIMRC Conference, Hague, vol. 4, pp. 1344-1347
Sep. 19-23, 1994. cited by other .
Yang, "Blind Joint Soft-Detection Assisted Slow Frequency-Hopping
Multi-Carrier DS-CDMA," IEEE Trans. Comm., vol. 48, No. 9, Sep.
2000. cited by other .
Hara, "Overview of Multicarrier CDMA," IEEE Communications Mag.,
Dec. 1997. cited by other .
Frenger, "A Parallel Combinatory OFDM System," IEEE Trans. Comm.,
vol. 47, No. 04, Apr. 1999. cited by other .
Saulnier, "Performance of an OPDM Spread Spectra Comm. System Using
Lapped Transforms," IEEE, 1997. cited by other .
Chang, "Wavelet-Based Multi Carrier CDMA for Personal Comm.
Systems," IEEE, 1996. cited by other .
Yee, "Multicarrier Code Division Multiple Access (MC-CDMA): A New
Spreading Technique for Comm. Over Multipath Channels," Final
Report for Micro Project 93-101. cited by other .
Xu, "Performance of Multicarrier DS CDMA Systems in the Presence of
Correlated Fading," IEEE, 1997. cited by other .
Sourour, "Performance of Orthogonal Multicarrier CDMA in a
Multipath Fading Channel," IEEE Trans. Comm., vol. 44, No. 3, Mar.
1996. cited by other .
Kowalski, "Optical Pulse Generation With a Frequency Shifted
Feedback Laser," Appl. Phys. Lett. 53(9), Aug. 29, 1988. cited by
other .
Kowalski, "Pulse Generation With an Acousto-Optic Frequency Shifter
in a Passive Cavity," Appl. Phys. Lett. 50 (12), Mar. 23, 1987.
cited by other .
Bonnet, "Dynamics of Self-Modelocking of a Titanium-Saphire Laser
With Intracavity Frequency Shifted Feedback, " Optics Comm. 123
(1976) Feb. 1, 1996. cited by other .
Bingham, "Multicarrier Modulation for Data Transmission: An Idea
Whose Time has Come," IEEE Communications Mag., May 1990. cited by
other .
Slimane, "MC-CDMA With Quadrature Spreading Over Frequency
Selective Fading Channels," IEEE, 1997. cited by other .
Yee, "Multicarrier CDMA in Indoor Wireless Radio Networks," IEICE
Trans. on Comm., Japan, vol. E77-B, No. 7, Jul. 1994. cited by
other.
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Primary Examiner: Leung; Christina Y
Attorney, Agent or Firm: Shattil; Steven J
Parent Case Text
This application claims priority to U.S. Provisional Application to
Ser. No. 60/163,141, filed Nov. 2, 1999, and is a
Continuation-In-Part of U.S. patent application Ser. No.
09/022,950, filed Feb. 12, 1998, now U.S. Pat. No. 5,955,992.
Claims
I claim:
1. A receiver system for a waveguide communication system for
receiving a transmitted multicarrier signal configured to map each
of a plurality of data symbols to a pulse in a pulse sequence
characterized by a superposition of carrier signals, including: a
multicarrier phase adjuster capable of providing phase adjustment
to received multicarrier signals, a combiner capable of combining
the multicarrier signals for producing a data-modulated pulse
sequence, and a time-domain receiver capable of producing data
estimates from the data-modulated pulse sequence.
2. The receiver system recited in claim 1 wherein the multicarrier
phase adjuster includes an optical-to-RF converter.
3. The receiver system recited in claim 1 wherein the multicarrier
phase adjuster includes a filter bank capable of separating the
received multicarrier signal with respect to frequency.
4. The receiver system recited in claim 1 wherein the multicarrier
phase adjuster is capable of providing a zero-phase relationship to
the received multicarrier signals.
5. A method of receiving a multicarrier signal being configured to
map each of a plurality of data symbols to a pulse in a pulse
sequence characterized by a superposition of carrier signals, the
method including: providing for adjusting the multicarrier signal
to compensate for multipath, providing for combining the
multicarrier signal to produce at least one time-domain signal
characterized by the plurality of data symbols modulated on a pulse
sequence, and providing for processing the at least one time-domain
signal to detect at least one of the plurality of data symbols.
6. The method recited in claim 5 wherein at least one of providing
for adjusting and providing for combining includes separating the
multicarrier signal into a plurality of carrier frequency
components.
7. The method recited in claim 5 wherein providing for adjusting
includes sampling the multicarrier signal.
8. The method recited in claim 5 wherein the superposition of
carrier signals comprises at least one of a TDMA signal, a DS-CDMA
signal, an MC-CDMA signal, an FHSS signal, and an OFDM signal.
9. The method claim 5 wherein providing for adjusting and providing
for combining are implemented with a matched filter.
10. The method recited in claim 5 wherein providing for adjusting
and providing for combining are adapted to project the multicarrier
signal onto at least one orthonormal basis.
11. The method recited in claim 5 wherein at least one of providing
for adjusting and providing for combining is adapted to compensate
for channel distortion.
12. The method recited in claim 5 wherein providing for combining
includes performing at least one of a set of combining processes,
including co-phasing, selective combining, maximal-ratio combining,
equal-gain combining, and maximal-selection combining.
13. The method recited in claim 5 wherein at least one of providing
for adjusting, providing for combining, and providing for
processing is implemented digitally.
14. A multi phase-space detector capable of detecting a plurality
of information signals modulated on a plurality of signal
phase-spaces that map each of the plurality of information signals
to a pulse in a pulse sequence characterized by a superposition of
carrier signals, the detector including: a coupler coupled to a
communication channel, the coupler capable of coupling a plurality
of transmitted signals out of the channel to produce coupled
signals, a frequency sampler capable of separating the coupled
signals into a plurality of frequency components, and a combining
circuit capable of combining the plurality of frequency components
to generate at least one pulse sequence modulated with the
plurality of information signals.
15. The multi phase-space detector recited in claim 14 wherein the
coupler includes an antenna array.
16. The multi phase-space detector recited in claim 14 wherein the
frequency sampler is implemented with a filter bank.
17. The multi phase-space detector recited in claim 14 wherein the
frequency sampler includes a signal processor adapted to perform at
least one Fourier transform.
18. The multi phase-space detector recited in claim 14, further
comprising a decoder.
19. The multi phase-space detector recited in claim 14, further
comprising at least one N-point invertible transform.
20. The multi phase-space detector recited in claim 14 wherein the
combining circuit includes at least one decision module adapted to
perform at least one of multi-user detection and multi-channel
detection.
21. The multi phase-space detector recited in claim 14 wherein the
combining circuit is adapted to perform at least one of a set of
combining processes, including co-phasing, selective combining,
maximal-ratio combining, equal-gain combining, and
maximal-selection combining.
22. The multi phase-space detector recited in claim 14 wherein at
least one of the frequency sampler and the combining circuit are
implemented with a digital signal processor.
Description
FIELD OF THE INVENTION
This invention relates generally to guided-wave optical
communications and specifically to multicarrier
optical-transmission protocols, such as Wavelength Division
Multiplexing (WDM). The application of the invention is directed
toward providing a multicarrier protocol that can be used for both
waveguide and wireless communications.
BACKGROUND OF THE INVENTION
In WDM, optical carriers having different wavelengths are
individually modulated. The transmission capacity of an
optical-fiber transmission line is increased according to the
number of wavelengths, which define WDM channels. A large number of
WDM channels can be supported if the channels are closely spaced
(e.g. 50 GHz).
WDM channels are transmitted over a single waveguide and
demultiplexed by diffraction or filtering such that each channel
wavelength is routed to a designated receiver. Optical amplifiers
(such as doped fiber amplifiers) simultaneously amplify the optical
channels, facilitating use of the WDM protocol.
Dense WDM systems require special add/drop multiplexer filters (ADM
filters) to add and drop channels (wavelengths). At each node in
the system, certain wavelengths on one fiber may be dropped onto a
second fiber, and channels from a third fiber may be added. The
number of WDM channels determines the number of nodes. However,
dispersion, four-wave mixing, wavelength drift of transmission
sources, and the difficulty of separating closely spaced
wavelengths at a receiver restrict the number and spacing of the
channels.
I. Dispersion
The principal limiting factor in high-rate communication systems is
chromatic dispersion. Chromatic dispersion is characterized by a
widening in the duration of pulses as they travel through a fiber.
Dispersion is caused by the dependence of the effective index of
the fiber on the wavelength of each wave transported. The variation
in the index of refraction with respect to wavelength causes
different channel wavelengths to travel at different speeds. This
phenomenon is also known as group-velocity dispersion (GVD).
Increasing data-transmission rates severely limits the transmission
distance because of the waveform distortion caused by GVD in
optical fibers. Furthermore, when the transmission speed is
increased, the optical power for transmission needs to be increased
to maintain the required received optical-power levels.
Many techniques and devices have been devised to counter the
effects of GVD. The goal of dispersion compensation is to change a
nonlinear channel into a linear channel (at least for a specific
range of wavelengths) in order to achieve the capacity of a linear
channel. None of these references make use of the nonlinearity of
the channel to achieve capacity that may exceed the theoretical
limitation of a linear channel.
U.S. Pat. No. 4,677,618 describes a method of compensating for
distortion of WDM data by a dispersive medium.
Dispersion-compensation techniques include providing lengths of
dispersion-compensating line in an optical network (U.S. Pat. No.
5,361,234) and providing dispersion-slope compensation, such as
disclosed by J. A. R. Williams et al. in IEEE Photonics Technology
Letters, Vol. 8. p. 1187 (1996) and K. Takiguchi et al. in
Electronics Letters, Vol. 32 p. 755 (1996). However, these
dispersion-compensation methods have relatively limited
effectiveness with respect to bandwidth.
To minimize the dispersion value of optical signals in fiber, work
is currently under way to transmit signals in the 1.55-.mu. range
in a dispersion-shifted fiber. U.S. Pat. Nos. 5,943,151, 5,898,714,
5,877,879, and 5,828,478, describe methods of phase comparisons and
synchronization to compensate for chromatic dispersion.
II. Other Distortions
Other types of distortion also occur. U.S. Pat. No. 5,847,862
describes shaping of amplifier outputs to offset depletion of
high-frequency channels. A significant factor in signal-to-noise
ratio (SNR) degradation in WDM is due to Raman crosstalk. U.S. Pat.
No. 5,953,140 cancels out the effects of crosstalk by processing
signals in the electrical domain after the WDM transmission has
been demultiplexed. Also, smoothing the power variation of the
optical signal transmitted through the fiber can reduce nonlinear
effects. U.S. Pat. No. 5,589,969 addresses the problem of
interference caused by four-wave mixing between different WDM
signals by providing a non-periodic spacing between the signal
wavelengths.
III. Wavelength Drift
WDM lasers require extremely tight manufacturing tolerances with
respect to center wavelength and line width. There are significant
problems with laser-wavelength drift resulting from environmental
factors, such as temperature variations and aging. Wavelength drift
causes substantial problems in distributed systems because each
receiver needs to demultiplex signals from different transmitters
and from different fiber lines, all of which independently operate
under different and changing environmental conditions. Conventional
WDM systems require strict manufacturing and environmental controls
to stay within tolerance.
Efforts to improve WDM systems have focused on improving the
wavelength stability of the transmitter lasers. U.S. Pat. No.
5,943,152 describes a method for stabilizing the wavelength of an
optical source. U.S. Pat. No. 5,838,470 addresses the problem that
WDM transmitters and receivers must be precisely tuned to
predetermined fixed wavelengths. In the '470 patent, each
transmitter transmits a synchronization signal that the receiver
uses to determine the wavelength of the signals. These signal
wavelengths are stored in a lookup table.
U.S. Pat. No. 5,894,362 includes a decoupling unit for decoupling a
portion of the WDM signal from a fiber as a monitoring signal. A
monitoring unit determines the spectrum of the WDM signal with an
optical spectrum analyzer. The monitoring unit uses the spectrum
information to control light sources such that the wavelength is
constant for each signal. The monitoring unit also detects SNR and
signal power, maintains received power levels, counts the number of
channels in a WDM signal, measures the spacing between wavelengths,
monitors any changes in the spectrum, and controls optical
amplifiers to achieve a desired noise figure or maintain a flat
gain.
U.S. Pat. No. 5,555,086 describes a sensor array used to monitor
physical characteristics of an optical fiber. A two-mode signal is
sent through the fiber. Each mode has a different propagation
velocity to create an interference pattern at a sensor array.
Changes in the interference pattern indicate changes in the
physical characteristics of the fiber.
None of the references disclose a communication protocol wherein
optical switching is controlled by relative frequencies and phases
between multiple carrier signals. None of the references disclose
an optical source that generates multi-frequency carrier signals
that are identically affected by frequency drifts. None of the
references describe a method of generating information signals from
relative frequency and phase relationships between carrier signals
in order to reduce the effects of carrier-frequency drifts.
IV. Insertion Loss
Insertion loss limits the number of sources that can be coupled
into a fiber. Insertion loss limits the number of wavelength
channels produced by multiple optical sources. U.S. Pat. No.
5,589,969 describes an array of passive resonant cavities
(Fabry-Perot filters) that reduces insertion loss. The cavities can
be used to demultiplex several received signal wavelengths and
multiplex several different wavelengths into a single multichannel
laser signal.
V. Switching
Virtual point-to-point connections are achieved with WDM when
different frequency channels are routed to different locations.
However, the number of locations is limited to the number of WDM
channels.
Wavelength demultiplexers with the smallest channel spacing are the
most difficult to fabricate. Small channel spacing also results in
cross talk between channels. U.S. Pat. No. 5,680,490 uses a
multi-stage array of demultiplexers and bandpass filters. The array
still requires the use of expensive demultiplexers to separate
closely spaced channels, and it increases the number of
demultiplexers.
U.S. Pat. No. 5,778,118 describes the use of optical add-drop
multiplexers for WDM systems. U.S. Pat. No. 5,854,699 discloses a
switching technique involving the modulation of data and control
signals onto the same optical carrier. U.S. Pat. No. 5,940,208
discloses a switchable fiber-optic device.
None of the references describe an interference relation between
different carriers that can be used to receive baseband information
signals.
VI. Frequency-Shifted Feedback
In the Optics Letters article "Broadband Continuous Wave Laser,"
applicant described a laser design that utilizes a traveling-wave
frequency-shifted feedback cavity (FSFC) to circulate light through
a gain medium. Light circulating through the FSFC is frequency
shifted by an acousto-optic modulator (AOM) upon each pass through
the cavity. A unique characteristic of this cavity is that, unlike
a Fabry-Perot cavity, it does not selectively attenuate signal
frequencies. In the thesis "A New Method for Generating Short
Optical Pulses," applicant describes how an optical signal
propagating through an FSFC is spread in frequency to generate
broadband lasing. The amount of frequency spreading is proportional
to the number of times that light circulates through the cavity. In
the Applied Physics Letters article "Optical Pulse Generation with
a Frequency Shifted Feedback Laser," applicant describes an
interference condition in which the broadband output of a laser
produces short optical pulses, which have a frequency that is
related to the RF-shift frequency of an AOM.
VII. Carrier Interferometry
In U.S. Pat. No. 5,955,992, U.S. patent application Ser. No.
09/393,431, and PCT Pat. Appl. No. WO99/41871, which are hereby
incorporated by reference, applicant describes a multicarrier
protocol that uses interference characteristics between the
carriers to convey baseband information signals. This protocol is
known as Carrier Interference Multiple Access (CIMA). The CIMA
protocol involves different carriers that are redundantly modulated
and selected to provide a predetermined phase relationship.
However, this redundancy does not diminish the bandwidth
efficiency. In fact, superior bandwidth efficiency is achieved
because the CIMA carriers combine in the time domain to produce
short impulses. This results in orthogonality in the time
domain.
In wireless systems, CIMA signals can be processed as both
low-bandwidth and high-bandwidth signals simultaneously. This
mitigates the effects of both intersymbol interference and
multipath fading. The frequency-diversity of CIMA signals greatly
reduces transmission power requirements and eliminates the effects
of jamming interference. CIMA also simplifies multi-user detection.
Unlike other spread-spectrum techniques that distribute user
interference to all users in a communication system, CIMA confines
user interference to adjacent users in the time-domain, thus making
it simple to cancel. CIMA can also be used to construct other
protocols, such as Direct-Sequence CDMA (DS-CDMA). However, unlike
DS-CDMA, which requires fast serial processing to create a
direct-sequence spreading code, CIMA uses a slow type of parallel
processing to generate the exact same codes (but with all of the
additional benefits of a CIMA signal). Slow parallel processing
makes CIMA systems simpler and less costly to implement.
Another benefit of CIMA is the ability to transmit and receive
signals in non zero-phase space, which is an interference condition
at one or more time instants within a symbol interval in which the
carrier signals corresponding to one data symbol cancel where
similar carrier signals corresponding to a different data symbol
constructively combine. CIMA signals are detectable by receivers
that are tuned to the carrier frequencies and carrier-phase
relationships of the carrier signals.
Frequency diversity in the CIMA protocol also enables spatial
demultiplexing of the received CIMA signals without antenna arrays.
Different carriers have different spatial gain distributions (in a
multipath fading environment) due to their differences in
frequency. Therefore, each transmitted signal has a unique spatial
gain distribution represented by the individual complex amplitudes
of its component carriers. A training sequence can be used to
determine (and adjust) the spatial gain distributions of the CIMA
carriers at one or more receivers. Received CIMA signals are then
separated into their carrier components. The complex amplitude of
each component includes spatial-gain terms and unknown information
signals. The amplitudes are extracted from each carrier and
processed in a cancellation system (described in U.S. patent
application Ser. No. 08/279,050, PCT Appl. No. WO95/03686, U.S.
patent application Ser. Nos. 08/862,859, 09/324,206, and
09/347,182, which are all incorporated by reference), which solves
for the unknown signals.
VIII. Redundancy
The transmission protocol used in the present invention is related
to CIMA. Although redundancy in transmission dimensions (such as
frequency) is well known in the prior art, CIMA is unique because
it provides benefits of frequency diversity via redundant
transmissions in the frequency domain without sacrificing bandwidth
efficiency.
U.S. Pat. No. 5,940,196 describes an optical-communication system
that transmits the same information on two optical carrier signals
having different frequencies and combines the signals at a receiver
to increase the carrier-to-noise ratio. However this benefit is
achieved at the expense of reducing bandwidth efficiency.
IX. Time-Division Multiplexing
The method of generating time-domain CIMA signals is distinct from
Optical Time Division Multiplexing (OTDM). U.S. Pat. Nos. 5,654,812
and 5,331,451 describe OTDM, which involves using time-domain
processes to generate information pulses modulated onto a carrier
signal. Each carrier signal is transmitted and optically
demultiplexed at a receiver.
X. Pulse Formation
An ideal optical pulse consists of an envelope of a fixed-frequency
carrier that is modulated according to a given temporal profile. G.
P. Agrawal (Nonlinear Fiber Optics, Academic Press Inc. section 3,
paragraph 2, pages 54 64) shows that in the absence of chirping,
the spectral amplitude is minimized with respect to the pulse
duration. Chirping is a frequency variation of the optical carrier
enveloped by the laser-generated pulse.
Soliton transmission techniques are used to mitigate dispersion.
Soliton-type pulses cause nonlinear variations in the refraction
index of an optical medium due to the high power of the solitons,
resulting in a counteracting of the effects of chromatic
dispersion. The objective of time-domain dispersion-compensation
techniques is to reduce the amount of pulse spreading in an optical
fiber.
None of the prior-art references teach a method for using
dispersion to reduce the duration of a pulse as it propagates
through an optical medium.
SUMMARY OF THE INVENTION
A first object of the invention is to provide a bandwidth-efficient
communication protocol that is appropriate for both waveguide and
wireless communications. A protocol that is common to waveguide and
wireless systems will facilitate local-access services and other
applications that require transmissions to be converted from
waveguide to free space and from free space to waveguide.
The first object is accomplished by using a redundant multicarrier
protocol. CIMA is a redundant multicarrier protocol proposed for
wireless applications. The CIMA protocol enables superior bandwidth
efficiency, interference rejection, frequency diversity, power
efficiency, and security compared to any other wireless protocol.
CIMA can also be used to create other protocols, such as CDMA.
A redundant multicarrier protocol (such as CIMA) used in a
waveguide provides superior performance compared to WDM. The
benefits of a redundant multicarrier protocol used in a waveguide
communication system include substantially increased capacity,
simplified modulation schemes, reduced insertion loss from coupling
transmit sources to the waveguide, a large number of channels,
simplified switching, and reduced receiver complexity.
A benefit of using a redundant multicarrier protocol in a
communication system that comprises both waveguide and wireless
systems is that a protocol conversion is unnecessary when the
transport medium changes from waveguide to free space (or free
space to waveguide). Consequently, a second objective of the
invention is to provide a communication system that uses the same
protocol for both waveguide and wireless communications. The CIMA
protocol enables conventional wireless protocols, such as CDMA, to
be constructed from multiple carriers. Therefore, conventional
wireless protocols can be conveyed through waveguides with the
benefits of the most efficient waveguide transmission protocol.
A third objective of the invention is to provide a protocol that
has the best performance of both waveguide and wireless protocols.
The third objective is accomplished by using CIMA.
A fourth objective is to provide a waveguide communication system
that uses a single optical source to generate and modulate multiple
carriers that are inserted into the waveguide. The importance of
using a single source coupled to the waveguide is that it reduces
insertion loss. The fourth objective is accomplished by using a
multicarrier or multimode signal source (such as a
frequency-shifted feedback or mode-locked laser) to generate
redundant multicarrier transmissions. A redundant multicarrier
protocol allows time-domain modulation of the combined signal
output by the signal source. In contrast, WDM requires that each
carrier be modulated separately, thus requiring the coupling of
multiple modulated carriers into the waveguide.
A fifth objective of the invention is to provide a multicarrier
waveguide communication system with a large number of carrier
signals having closely spaced wavelengths. Practical
implementations of WDM are limited to a few dozen wavelengths. In
WDM, 50 GHz between carrier frequencies is considered to be a close
spacing. The CIMA protocol provides for larger numbers of closely
spaced carrier signals. A laser with an FSFC can be used to
generate optical CIMA signals. An FSFC laser ("Optical Pulse
Generation with a Frequency Shifted Feedback Laser," Applied
Physics Letters, 1988) can generate at least 6000 carrier signals
having a frequency spacing of 110 MHz.
A sixth objective of the invention is to use nonlinearity (such as
chromatic dispersion) in the waveguide to increase transmission
capacity. The invention achieves the sixth objective by exploiting
the superposition of multi-frequency carrier signals in a
dispersive medium. The phase relationship between two or more
signals propagating in a dispersive medium changes with respect to
differences in velocities of the carrier signals. When the signals
are in phase (a zero-phase condition), the superposition of the
signals produces a maximum. As the signals move out-of-phase (a non
zero-phase condition), the superposition of the signals drops to
zero.
The invention includes methods and apparatus for transmitting
multicarrier signals having a predetermined phase relationship upon
insertion into a dispersive waveguide. As the signals propagate in
the waveguide, they combine in phase at one or more predetermined
locations. These locations may be nodes. The signals combine
destructively at other nodes, thus being undetectable or minimally
detectable by receivers at those nodes. Therefore, the same
carrier-signal bandwidth may be exploited at multiple locations
along a dispersive waveguide without providing significant
interference at any of the other locations. Receivers may be
responsive to more than one phase relationship between received
carriers in order to make use of the greater bandwidth provided by
the nonlinear waveguide. The bandwidth reuse factor can approach
the number of carrier frequencies used in the waveguide.
A seventh objective of the invention is to provide a substantial
increase in the number of virtual channels in a waveguide
communication system. The number of virtual channels in a WDM
system is limited to the number of carrier signals. Because this
number is relatively small, add/drop switches are required to
provide frequency (channel) reuse in the WDM system.
The seventh objective is accomplished by making use of phase
relationships between individual carrier signals to define
different channels. A virtual switch is enabled by the phase
relationship imparted to the carriers by the transmitter. The phase
relationship defines the destination of the information signals
modulated onto the carriers because it determines the location(s)
in the waveguide where the signals combine in phase to produce a
detectable signal. The number of virtual switches (destinations)
may be at least the number of carrier frequencies, which can exceed
6000.
An eighth objective of the invention is to eliminate the
requirement for stable optical sources (and associated
wavelength-control systems) in an optical communication system.
This objective is also related to a ninth objective, which is to
eliminate the need for wavelength demultiplexers at receivers in
the system. The eighth and ninth objectives are achieved by at
least one embodiment of the invention in which a redundant
multicarrier protocol is used. Information signals conveyed using
CIMA depend on the relative phase and the frequency separation of
the carriers. The carrier frequencies can drift without changing
the relationships that affect the information signal. If the
dispersion characteristics of the carrier signals are not changed
significantly, the destination(s) of the information signal will
not change. A simple receiver may be used at one or more
destinations to detect a constructive-interference information
signal. The simple receiver comprises an envelope detector and does
not require a wavelength demultiplexer.
A tenth objective of the invention is to provide a multi-user
detection scheme to reduce interference and increase capacity. The
tenth objective is achieved in one aspect of the invention by the
process of time-domain processing of the received signals.
Reception of signals in adjacent nodes may be performed (either by
direct detection in adjacent nodes or dispersion-compensation
shifting of received signals at one node) to provide
interference-cancellation signals. The cancellation signals are
applied to a desired signal to cancel the effects of multi-user
interference.
The tenth objective is also achieved by a second aspect of the
invention, which exploits frequency diversity in multiple received
signals to cancel or otherwise separate interfering signals.
Variations in the complex amplitude of each carrier signal results
from propagation effects, such as reflections, dispersion, and
frequency-selective attenuation. Therefore, different transmitter
locations along the waveguide may result in distinct
frequency-versus-amplitude profiles of the transmitted signals
received by a receiver. Transmitters may also apply amplitude
variations to the transmitted signals to help receivers determine
the source(s) of received transmissions. Training sequences or
estimation techniques may be used to determine the relative
amplitude of the frequency components of the different transmitted
signals at one or more receivers. Knowledge of the relative
amplitudes can be used to separate unknown information signals from
the received signals. A cancellation method and/or a constellation
technique may be performed to determinate the values of the unknown
signals.
A variation of the second aspect of the invention involves using
receivers at multiple receiver sites. The frequency-diversity
method using cancellation can explicitly solve for a number of
unknowns that equals the number of carrier frequencies multiplied
by the number of receivers. The receivers each have a different
location along the waveguide. Using one of the cancellation
methods, each receiver generates a set of equations having a number
of unknowns (typically equal to the number of carrier frequencies).
If the number of unknowns is greater than the number of equations,
then the equations generated by multiple receivers are combined to
produce a number of algebraically distinct equations that is
greater than or equal to the number of unknowns.
The multi-receiver frequency-diversity technique may also be
implemented in a free-space communication system and method. In a
wireless communication system, spatially separated wireless
receivers (such as antennas) are used instead of the waveguide
receivers.
An eleventh objective of the invention is to provide a security
protocol that makes communications in both waveguides and free
space difficult to intercept. The invention achieves the eleventh
objective by transmitting phase-based destination information on
carrier signals that cancel in a waveguide at locations other than
the intended destination. In free space, non-zero phase-space
transmissions are not detectable by conventional receivers. The
inability to detect a signal makes it difficult to intercept.
A twelfth objective of the invention is to provide cancellation of
periodic-phase carriers in order to increase system capacity. The
twelfth objective is accomplished by an interferometry switch,
which transmits a cancellation signal to null an undesired signal.
A transmission received by a receiver can be used to generate a
cancellation signal that nulls any residual signal levels of the
received signal that would otherwise occur farther down the
waveguide. The cancellation signal may be transmitted over a second
waveguide that is shorter than the primary waveguide to compensate
for delay between reception of the desired signal and transmission
of the cancellation signal. The cancellation signal may be
transmitted at a different (lower) frequency band that has a higher
propagation velocity. A receiver that receives a frequency-shifted
cancellation signal may convert the signal back to its original
frequency. The cancellation signal may be transmitted over a
wireless link.
The objectives of the present invention recited above, additional
objects, and/or alternative objects depend on particular
embodiments and applications of the invention, and are apparent in
the description of the preferred embodiments.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic of a high-capacity optical-fiber network that
provides last-mile information delivery to individual users via
wireless links.
FIG. 2 is an illustration of basic components of a transmitter that
generates redundantly modulates carrier signals.
FIG. 3A is a plot of a plurality of multi-frequency carriers and a
superposition of the carriers.
FIG. 3B is a plot of a square-wave time-domain pulse.
FIG. 3C is a frequency-domain plot of the square-wave pulse shown
in FIG. 3B.
FIG. 3D is a time-domain plot of a CIMA pulse.
FIG. 3E is a frequency-domain plot of the CIMA pulse shown in FIG.
3D.
FIG. 3F is a flow diagram representing a method of generating CIMA
signals.
FIG. 3G is a schematic of a CIMA receiver.
FIG. 3H is a schematic of a CIMA receiver having a single matched
filter.
FIG. 4A is a plot of a composite signal resulting from a
superposition of carrier signals having an equal frequency
spacing.
FIG. 4B is a plot of a composite signal resulting from a
superposition of carrier signals having carrier frequencies that
are unequally spaced.
FIG. 5A is a plot of relative amplitudes of a set of
multi-frequency carriers that produce a composite signal 130 (shown
in FIG. 5B) having time-domain characteristics of a direct-sequence
CDMA signal.
FIG. 5B is a time-domain plot of a superposition of the carrier
signals shown in FIG. 5A.
FIG. 6A is a diagram of a transmission method performed by a
multicarrier transmitter.
FIG. 6B is a flow diagram of the steps performed by an alternative
embodiment of a multicarrier transmitter.
FIG. 7 is a diagram of a multicarrier modulator.
FIG. 8 is a plot of index of refraction n.sub.index with respect to
signal wavelength .lamda. in an optical fiber.
FIG. 9 is a plot of relative positions of two waves having
different wavelengths at different locations in a dispersive
medium.
FIG. 10 is a diagram that illustrates a process of addressing
signals in a dispersive medium by showing the pulse widths and
pulse heights of signals having different addresses.
FIG. 11A is a schematic of a multicarrier transmission system that
provides addressing to transmitted signals.
FIG. 11B is a flow diagram of an addressing method for addressing
signals in a dispersive medium.
FIG. 12 shows a general form of a multicarrier receiver.
FIG. 13 is a functional diagram of a receiver and reception
method.
FIG. 14A is a diagram of a receiver for an optical system.
FIG. 14B is a schematic for an optical receiver.
FIG. 14C is a diagram of a receiver having a signal-component
adjuster
FIG. 14D is a diagram of a multi-channel receiver.
FIG. 14E is a schematic of a receiver that performs address
separation.
FIG. 15 is a diagram of a transport-medium interface that couples
received optical signals from an optical waveguide to a free-space
propagation environment.
FIG. 16 is a flow diagram for a process of transmitting signals
that transition between a waveguide and a wireless channel without
requiring a change in protocol.
FIG. 17A is a schematic of a waveguide communication system that
includes a cancellation line to cancel signals received at a
desired receiver.
FIG. 17B is a diagram of an alternate embodiment of a waveguide
communication system that cancels signals after a desired receiver
receives them.
FIG. 17C is a flow diagram that describes a process for canceling
communication signals in a communication channel.
FIG. 18 is a process diagram for a diversity-based cancellation
system.
FIG. 19 is a time-domain representation of a plurality of
frequency-domain encoded signals.
FIG. 20A is a process diagram describing the functions performed by
a multiple-diversity communication system.
FIG. 20B is an alternative process diagram for a multiple-diversity
communication system.
FIG. 21A is a graphical representation of two different types of
diversity processing.
FIG. 21B is a graphical representation of a hybrid form of
diversity processing that makes use of at least two forms of
diversity processing.
FIG. 22A is a schematic of a frequency-diversity interferometry
multiplexing system.
FIG. 22B is a diagram of a cascaded interferometry system that uses
frequency-diversity interferometry multiplexing and spatial
interferometry multiplexing.
FIG. 23 is a diagram of a spread-spectrum interferometer.
FIG. 24 is a schematic of a redundant-carrier communication system
in which a plurality of carriers are received and separated with
respect to at least one diversity parameter and then processed and
combined with respect to another diversity parameter.
FIG. 25A is a process diagram that outlines a method of
communication that uses redundantly modulated multicarrier
signals.
FIG. 25B is a process diagram that describes another method of
communication that uses redundantly modulated carrier signals.
FIG. 25C is a process diagram that outlines a method of receiving
communication signals that include redundantly modulated carrier
signals.
FIG. 26 is a schematic diagram of a communication system that uses
redundantly modulated carriers to enhance diversity and uses
spatial interferometry multiplexing to increase capacity.
FIG. 27 is a schematic of a receiver 200 that receives multicarrier
signals and achieves benefits of spatial diversity, frequency
diversity, and capacity enhancement of spatial interferometry
multiplexing.
FIG. 28A is a diagram of a receiver 200 that receives a deficient
number of receive signals 265 (i.e., equations) and applies a
nonlinear process 266 to at least one of the equations to generate
one or more additional algebraically unique equations.
FIG. 28B is a process diagram that shows steps for separating
unknown signals in a plurality of received signals.
FIG. 29 is a process diagram that describes a method of solving M
linear equations having N unknown signals where M<N.
FIG. 30 is a diagram of a base station and a plurality of
transceivers that may use a combination of differential modulation
and spatial interferometry multiplexing to achieve enhanced system
capacity.
FIG. 31A is a plot of SNR for signals separated from a 3-element
array for different levels of multi-user interference.
FIG. 31B is a plot of SNR for signals separated from a 4-element
array for different levels of multi-user interference.
FIG. 32 is a plot of a two-dimensional signal space comprised of a
spatial dimension and a differential power dimension.
FIG. 33 is a process diagram that shows a method of calibrating
weights in a multi-user detector.
FIG. 34 is a process diagram that shows a looped optimization
process.
FIG. 35 is a process diagram that outlines a method of adjusting
reception parameters and assigning transmissions to signal spaces
in order to optimize system-operating parameters.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
The following description is directed toward the implementation of
an optical-fiber network having a wireless interface at each node.
The implementation of the invention can be directed generally to
waveguide and wireless applications.
FIG. 1 shows the basic components of a high-capacity optical-fiber
network that provides last-mile information delivery to individual
users via wireless links. A plurality of couplers 150A, 150B, 150C,
and 150D have different locations in a communication channel 99
that supports the propagation of electromagnetic communication
signals.
A coupler (such as couplers 150A, 150B, 150C, and 150D) couples
electromagnetic signals into the communication channel 99 or
couples electromagnetic signals out of the communication channel
99. Couplers can include lenses, antennas, any type of
electromagnetic-wave radiator, and any type of electromagnetic-wave
receptor. A coupler may be a directional coupler.
The communication channel 99 is any type of transport medium for
electromagnetic waves used for communications. Any kind of
electromagnetic wave, such as optical (including infrared) and RF
(including microwave), may be used for communication. The channel
99 may be a free-space propagation environment, a guided-wave
environment, or both. The channel 99 may cause signal distortion
and intersymbol (or intercede) interference.
The material through which the communication signals propagate, the
shape and dimensions of the material, and the mode of transport
defines a transport medium. The characteristics of the propagation
environment can be represented by electrical characteristics, such
as resistance, inductance, and capacitance. Different types of
waveguides represent different transport mediums. Different modes
of transport, such as guided wave and free space (i.e., wireless),
define different transport mediums.
The communication channel 99 shown in FIG. 1 is an optical fiber,
which is a type of waveguide. Other types of waveguides include
transmission lines (such as coaxial, microstrip, and twisted pair),
parallel plate, channelized free space, and any linear or nonlinear
medium that directs propagation of electromagnetic waves by
refraction or reflection.
Each coupler 150A, 150B, 150C, and 150D in FIG. 1 is coupled to a
transmitter 100A, 100B, 100C, and 100D and a receiver 200A, 200B,
200C, and 200D. One or more of the couplers (such as couplers 150A,
150B, 150C, and 150D) may couple only a transmitter or a receiver
to the channel 99. One or more of the couplers (such as couplers
150A, 150B, 150C, and 150D) may couple additional parts of the
communication channel 99 (such as optical fibers) to the part of
the communication channel 99 shown in FIG. 1.
The receivers 200B and 200C are each coupled to a transmitter 100E
and 100F. The transmitters 100E and 100F are each coupled to a
coupler 150E and 150F. The couplers 150E and 150F provide an
interface to a wireless channel (not shown) and couple transmission
signals provided by the transmitters 100E and 100F into the
wireless channel (not shown). Each coupler 150E and 150F includes
an antenna 158E and 158F. The couplers 150E and 150F receive
signals from the wireless channel (not shown) and convey the
received signals to a receiver 200E and 200F. The receivers 200E
and 200F are coupled to the transmitters 100B and 100C, which
couple the received wireless signals into the optical fiber 99.
The uniqueness of the present invention shown in FIG. 1 is based,
in part, on the types of communication signals used. Consequently,
the design of the transmitters and receivers are also unique.
FIG. 2 shows a transmitter 100 of the present invention. A
multicarrier-signal generator 102 is coupled to an
information-signal modulator 104. The transmitter 100 may include a
coupler (not shown) to a communication channel (not shown). The
signal generator 102 may include signal-processing systems (not
shown), such as, but not limited to weighting systems, transform
generators, modulators, filters, and feedback systems. The signal
generator may include a polarization controller (not shown).
The signal generator 102 produces a multicarrier signal. A
multicarrier signal is defined as a plurality of carrier signals
having different orthogonalizing properties (also referred to as
orthogonality parameters or diversity parameters), such as time,
differential power, location, mode, frequency, polarization, phase
space, directivity, spread-spectrum code, or any combination of
orthogonalizing properties. An orthogonalizing property (such as
polarization) may not be completely orthogonal. For example,
polarized signals having less than 90-degrees separation between
them have cross-polarization (interference) terms. A multicarrier
signal may be defined by any signal property that affects
propagation characteristics, such as velocity, reflections, and
refraction. Each multicarrier signal may be defined by a different
propagation mode.
A multi-frequency signal generator is a type of multicarrier-signal
generator. A multi-frequency signal generator is any signal
generator that generates electromagnetic signals having
frequency-diverse characteristics, such as multiple signal
frequencies. Frequency-diverse signals may have diversity according
to other diversity parameters, such as time, location, mode,
polarization, or diversity parameters resulting from any other
orthogonalizing property. The multicarrier signals may have any
frequency in the electromagnetic frequency spectrum. However, for
optical waveguide applications it is assumed that the signals are
optical. In free-space applications, the signals are assumed to be
RF (including microwave) or optical (including infrared).
In this case, the signal generator 102 produces a plurality of
carrier signals having a plurality of frequencies. The signal
generator 102 includes a frequency-diverse transmission source (not
shown). A frequency-diverse signal may be a multicarrier,
broadband, frequency-hopped, or chirped signal. The transmission
source (not shown) may be any type of frequency-diverse
electromagnetic signal source, which may include mode-locked
lasers, laser arrays, FSFCs, frequency-shifted feedback lasers, or
broadband sources. A broadband source (not shown) may include a
wavelength demultiplexer (not shown) for separating continuous-wave
radiation into carriers having discreet frequencies or discreet
frequency bands.
The signal generator 102 may include any type of multi-frequency
optical source. Many optical sources disclosed in the prior art are
appropriate for the signal generator 102 and are incorporated
herein by reference: U.S. Pat. No. 5,881,079 describes a laser
cavity having a frequency-routing device comprising controllable
frequency-selective pathways to allow multiple lasing frequencies
to be supported. U.S. Pat. No. 5,936,752 describes a method of
coupling light from a broadband source into a wavelength
demultiplexer for creating discreet wavelengths. A broadband source
may be provided by U.S. Pat. No. 5,923,683, which discloses a
coherent source of white light. U.S. Pat. No. 5,347,525 describes a
mode-locked laser for providing multiple signal wavelengths. U.S.
Pat. Nos. 5,450,427 and 5,923,686 describe mode-locked lasers used
to create short pulses by either active or passive mode
locking.
Mode-locking lasers have a modulator in the laser cavity to provide
optical losses or gains at a frequency corresponding to the
separation frequency between two adjacent longitudinal cavity
modes. In active mode-locking lasers, the emitted pulse frequency
depends on the excitation frequency of the modulator. A mode-locked
laser may be a linear or ring-cavity laser. Active mode-locking
systems can produce a large number of locked pulses that
simultaneously travel through a ring cavity, and therefore enable
the pulse frequency to be much higher than in passive-type
devices.
Optical-fiber laser devices have an active electro-optical
modulation device in an optical path forming a laser cavity.
Harmonic mode locking occurs when the modulation frequency of the
device is an integer-valued multiple of the intermode separation
frequency. Harmonic mode locking is particularly useful in fiber
lasers because it enables shorter pulses to be produced.
FIG. 3A shows a composite signal 130 resulting from the
superposition of a plurality of multi-frequency carriers. The
superposition of multi-frequency carriers is described by U.S. Pat.
No. 5,955,992, which is hereby incorporated by reference. A
composite signal is defined herein as any superposition of a
plurality of signals.
The composite signal 130 may be a CIMA signal, which is a signal
comprised of carrier signals having predetermined frequency and
phase relationships. FIG. 3B shows a typical square-wave
time-domain pulse. FIG. 3C is a frequency-domain plot of the
square-wave pulse shown in FIG. 3B. FIG. 3D shows a CIMA pulse in
the time domain. A frequency spectrum corresponding to the CIMA
pulse is shown in FIG. 3E. FIG. 3D and FIG. 3E show that CIMA
signals may be added to either or both the frequency domain and the
time domain.
CIMA supports both orthogonal and pseudo-orthogonal waveforms.
Basic forms of CIMA can be used to double capacity in traditional
TDMA systems and simultaneously improve system performance in a
multipath environment. CIMA allows systems to achieve the benefits
of frequency diversity in which they previously could only benefit
from path diversity. Similarly, CIMA allows conventional DS-CDMA
systems to achieve the performance benefits of MC-CDMA.
FIG. 3F shows a method for generating CIMA signals. An information
signal s.sub.k(t) (from an input data source 101) intended for a
k.sup.th user is split and modulated onto N carriers in a first
modulation step 104A. In a second modulation step 104B, a plurality
of complex weights is applied to the modulated carriers. The
complex weights include phase shifts (or delays). Unlike a chip
sequence in MC-CDMA (which uses binary values, such as .+-.1), CIMA
signals use values of e.sup.in.DELTA..phi..sub.k. In an optional
third modulation step 104C additional weights a.sub.nk are applied
to the carriers. The weights a.sub.nk may include windowing
weights, channel-compensation values, and/or weight values that
facilitate signal separation by cancellation or constellation
methods at a receiver (not shown). The modulation steps 104A, 104B,
and 104C may be performed in any order and may be combined.
A CIMA signal corresponding to the superposition on N carriers
uniformly spaced in frequency by f.sub.s has a waveform envelope
according the equation:
.function..times..times..times..times..pi..times..times..times..function.-
.pi..times..times..times. ##EQU00001## The CIMA envelopes are
periodic with a period of 1/f.sub.s. The mainlobe of the envelope
has a width of 2/Nf.sub.s, and the N-1 sidelobe widths are
2/Nf.sub.s. Applying a phase shift of n.DELTA..phi..sub.k to each
n.sup.th carrier shifts the CIMA envelope in time by
.DELTA.t=.DELTA..phi..sub.k/2.pi.f.sub.s. Therefore N signals can
be positioned orthogonally in time. The phase shifts provide the
necessary phase relationships to create the desired timing of the
information signal received by at least one receiver (not
shown).
The cross correlation between users is:
.function..tau..times..times..function..times..times..times..pi..times..t-
imes..times..tau..function..times..times..pi..times..times..times..tau..ti-
mes..function..times..times..times..pi..times..times..times..tau.
##EQU00002## where .tau. is the time shift between envelopes. Zeros
occur at k/Nf.sub.s, k=1, 2, . . . , N-1 and (2k-1)/2(N-1)f.sub.s,
k=1, 2, . . . , N-1. CIMA can support N orthogonal users. If
additional users or signals need to be accommodated, CIMA provides
N-1 additional positions to place signals.
Modulated carriers may be combined in a combining step 109. The
combined signals may be up converted in an up-conversion process
205, which may include mixing with a carrier signal having a
frequency f.sub.c. The carrier signals are then coupled into a
communication channel 99 by a coupling process 150.
A CIMA signal has a number of carrier signals that may each have a
bandwidth that is less than the coherence bandwidth of the
communication channel. The coherence bandwidth is the bandwidth
limit in which correlated fading occurs. The total bandwidth of the
CIMA signal preferably exceeds the coherence bandwidth.
CIMA signals may be spaced in frequency by large amounts to achieve
a large system bandwidth relative to the coherence bandwidth. In
this case, CIMA signals make use of the frequency diversity
parameter to achieve uncorrelated fading. However, any diversity
parameter or combination of diversity parameters may be used to
achieve uncorrelated fading over the system bandwidth (or even
between individual carriers). For example, the system bandwidth of
a group of CIMA carriers may be defined by the coherence bandwidth
of one or more subchannels, such as spatial subchannels. Carriers
that are closely spaced in frequency may have uncorrelated fading
if they are transmitted from different locations or have different
values of directivity. CIMA carriers transmitted from different
locations may have different fades over each spatial subchannel and
therefore can benefit from diversity combining at a receiver (not
shown).
A CIMA receiver is shown in FIG. 3G. CIMA signals are coupled out
of a communication channel 99 by a coupler. Information signals are
separated from each carrier by a plurality of down converters, such
as down converters 205A, 205n, and 205N. The down converters 205A,
205n, and 205N may include a filter bank. In this case, the down
converters 205A, 205n, and 205N project the received signals onto
the orthonormal basis of the transmitted signals. Down converters
(such as the down converters 205A, 205n, and 205N) may additionally
compensate for channel distortion and/or addressing. The down
converters 205A, 205n, and 205N may apply windowing or other
filtering processes to the received signals.
Signals output from the down converters 205A, 205n, and 205N may be
sampled by a plurality of samplers 214A, 214n, and 214N before
being combined in a combiner 109. A decision device 255 detects the
combined signals. The decision device 255 may be part of the
combiner 109. The decision device 255 may perform multi-user
detection or multi-channel detection and may perform any
combination of cancellation and constellation processes to
determine the value(s) of received signals.
FIG. 3H shows a CIMA receiver having a single matched filter 134.
The matched filter 134 may optionally provide time limiting
(gating) to the received signals before being processed by a
decision device 255.
FIG. 4A and FIG. 4B show composite signals resulting from
superpositions of equally spaced carrier frequencies and unequally
spaced carrier frequencies, respectively. Equally spaced carrier
frequencies produce a composite signal that has periodic pulses.
Unequally spaced-in-frequency carriers produce a non-periodic
composite signal that has reduced sidelobe levels. In WDM, unequal
spacing of channels (with respect to wavelength) is desirable
because it reduces four-wave mixing.
Unequally spaced carrier signals refer to any type of sparse or
ultra-sparse spacing, such as referred to in array processing, but
applied to frequency or wavelength spacing of the carriers. Unequal
spacing includes random spacing, non-redundant spacing, or any type
of spacing determined by a nonredundant mathematical relation, such
as prime numbers, 2.sup.n relationships, or Fibonocci series.
The multicarrier-signal generator 102 or the modulator 104 may
provide a frequency-versus-amplitude windowing function to the
carrier signals. Windowing functions include spatially variant
apodization, and any other methods of reducing sidelobes, such as
described in U.S. Pat. No. 5,955,992.
The modulator 104 may modulate the carrier signals with an
information signal. The modulator 104 may use the carrier signals
to modulate the information signal. The modulator 104 may use any
type of modulation scheme, such as AM, FM, ASK, FSK, PSK, PAM, TOM,
Pulse Position Modulation, and any type of differential
modulation.
Information signals are communication signals that are unknown (or
have at least one unknown characteristic) at a receiver prior to
transmission. The information signal may be analog or digital. The
information signal may be a baseband information signal, an
information signal modulated with an intermediate frequency, or a
coded information signal that has been encoded with any combination
of encryption, error-correction, and spread-spectrum codes.
The modulator 104 may provide weights to each of the carriers
according to a predetermined code. The coded weights may be applied
to the carriers in order to generate a predetermined time-domain
profile, such as a direct-sequence signal. A preferred embodiment
of the invention includes a process of applying complex weights to
a multicarrier signal in order to create a predetermined
time-domain signal. A predetermined time-domain signal (or profile)
is defined herein as a specific shape of at least one signal
parameter, such as amplitude, frequency, polarization, and phase in
the time domain. The time-domain shape may be characterized by any
signaling protocol, such as TDMA and CDMA.
Although carriers may be modulated with respect to codes, or
information signals may be modulated onto the carriers within
multiple time intervals or having different time offsets, the
carriers are redundantly modulated with at least one information
signal. In this specification, redundantly modulated multicarrier
signals describe any of a set of signals wherein at least one
information signal is modulated onto a plurality of carrier signals
having different values of at least one diversity parameter. A
modulator may simultaneously modulate the carriers with the
information signal, or it may modulate each carrier independently.
The carrier signals may be modulated at different time intervals.
The carriers may be modulated with an encoded information signal.
The carriers may be modulated non-redundantly or quasi-redundantly
with spreading, error-correction, or encryption codes in addition
to being redundantly modulated with the information signal(s).
FIG. 5A shows the relative amplitudes of a set of multi-frequency
carriers that produce a composite signal 130 (shown in FIG. 5B)
having time-domain characteristics of a direct-sequence CDMA
signal. If the carriers are uniformly spaced in frequency, then the
time-domain signal is periodic. The number of direct-sequence chips
in the time-domain signal may be proportional to the number of
carrier signals. The bandwidth of the time-domain sequence is
related to the bandwidth of the carriers, which is related to the
number and spacing of the carriers.
FIG. 6A shows a multicarrier transmitter and transmission method of
the present invention. An n.sup.th information signal s.sub.n(t),
such as for an n.sup.th user, is applied to an n.sup.th code
c.sub.n having N chips c.sub.kn(k=0 to N-1). The information signal
s.sub.n(t) may modulate the code or it may be modulated by the code
c.sub.n.
A code generator 114 provides the N chips c.sub.kn to a
serial-to-parallel converter 107 that arranges the chips c.sub.kn
to parallel modulate each of a plurality of carrier signals
generated by a carrier-signal generator 102. If the carriers are
multi-frequency carriers, the carrier-signal generator 102 may be
represented by the operation or implementation of a digital method
for generating multi-frequency carriers, such as an inverse
Discreet Fourier Transform or an inverse Fast Fourier Transform.
Each chip c.sub.kn may be applied to a frequency bin of a transform
process. The chips c.sub.kn may have binary, real, or complex
values. The modulated carriers are optionally coupled to an output
processor 112, which processes the carriers prior to coupling them
into a communication channel (not shown).
The code generator 114 can be used as an information-signal encoder
or a carrier encoder. An information signal may be used to modulate
at least one code sequence. The code generator 114 may be a
multi-stage code generator. Code generators may include one or more
N-point transforms. N-point transforms include Discrete Fourier
Transforms (DFT), Fast Fourier Transforms (FFT), Walsh Transforms
(WT), Hilbert Transforms (HT), Randomizer Transforms (RT),
Permutator Transforms (PT), Inverse DFTs, Inverse FFTs, Inverse
WTs, Inverse HTs, Inverse RTs, Inverse PTs, and any other
reversible transform.
The output processor 112 may combine the carriers and/or provide
additional processing, such as filtering, interleaving, up
converting, down converting, coding, weighting, amplifying, and
mixing. A multicarrier signal may appear as a continuous broadband
signal (in the frequency domain) if the carriers are modulated by a
signal that has a large bandwidth with respect to carrier-frequency
separation.
A direct-sequence time-domain signal is produced by an appropriate
selection of chip values c.sub.kn. Therefore, the generation of the
periodic time-domain chip sequence does not require any time-domain
processing. The only time-domain processing involves the modulation
of the time-dependent information signals s.sub.n(t) onto the
carriers. This multicarrier method of generating CDMA signals
enables ultra-wideband CDMA to be deployed without the high-speed
processing requirements of conventional direct-sequence chip
generation.
FIG. 6B illustrates an alternative embodiment of a transmitter 100
of the invention. Some of the components shown in FIG. 6B may be
separate from the signal generator 102 and modulator 104 shown in
FIG. 2, or they may be integrated into those components 102 and
104. Therefore, the simplicity of the transmitter 100 illustrated
in FIG. 2 is meant to convey that there is a broad range of
transmitter designs that can be used to generate redundantly
modulated multicarrier signals. Furthermore, at least some of the
basic principles behind the transmitter's 100 operation are
applicable to transmitters that produce different types of
electromagnetic signals and transmitters that are coupled to
different kinds of communication channels.
An information-signal source 101 provides information signals
s.sub.n(t) to an encoder 108, which may digitize and code the
signals s.sub.n(t) to create bits of information s.sub.n(k). Signal
coding may include spread-spectrum, error-correction, or encryption
coding. Information bits s.sub.n(k) are represented as one form of
the information signal s.sub.n(t). The bits s.sub.n(k) are provided
to a modulator 104 which produces a plurality of modulated
symbols.
The modulated symbols are coupled to a predistortion device 111,
which adjusts signal parameters (such as power, gain-profile, and
phase) in order to compensate for distortion resulting from network
components (such as the channel, amplifiers, and receivers). The
predistortion device 111 is an optional part of the transmitter
100. The modulated symbols may be processed in a carrier-signal
generator 102, which may be a digital signal processor. The signal
generator 102 performs an inverse Fourier Transform, and may
perform other digital processing methods, such as filtering and
pulse shaping. A low-pass filter 113 may filter the output of the
signal generator 102. The modulated symbols may be mixed with a
carrier signal from a local oscillator 203. The local oscillator
203 may be used to either up convert or down convert modulated
carrier signals that are coupled into a communication channel by a
coupler 150.
FIG. 7 shows a diagram of a multicarrier modulator 104 for use in
an optical communication system. A frequency-diverse optical source
(not shown) provides a frequency-diverse signal to a wavelength
division multiplexer 103 that separates the frequency-diverse
signal into a plurality of frequency components representing
individual carrier signals. Each carrier is coupled through one of
a plurality of carrier modulators 105A to 105E. Modulation signals
are provided by a modulation signal generator 107 coupled to each
of the carrier modulators 105A to 105E. The modulated carriers may
be combined by a combiner 109 before being coupled into a
communication channel (not shown).
A combiner, such as the combiner 109 is any device or process that
has an input of a plurality of signals and an output representing a
superposition of the signals. A combiner may be a physical device,
such as a wavelength multiplexer, splitter, voltage divider, a
summer, or a difference amplifier. A combiner may be a combining
process performed by a computer processor. A combiner may provide
phase shifts to one or more input signals, filtering, inversion,
interleaving, de-interleaving, or amplitude adjustment prior to
combining the input signals.
Any of the modulators 104 used in the invention may include a
selective modulation unit, such as the modulator disclosed in U.S.
Pat. No. 5,949,925 that operates on each carrier signal
individually.
U.S. Pat. No. 5,796,765 (which is incorporated herein by reference)
describes a mode-locked laser used to control an optical switch.
The laser has an intracavity modulator that is repetitively
modulated at an integer multiple of the cavity round-trip time.
Output pulses are in bit positions that correspond to the signal
input to the modulator. This method could be used to switch the
transmission of a CIMA signal output to an optical fiber. This
laser could also be used to generate the CIMA signals, and in the
process control the timing of the CIMA signals. U.S. Pat. No.
5,812,302 discloses a high-speed frequency-modulation signal
source.
Pulsed signals may be digital or analog modulated. The modulator
104 may be a Mach-Zender modulator made of lithium niobate
(LiNbO.sub.3). Lithium niobate external modulators are typically
used to provide amplitude-shift key or phase-shift key modulation.
Frequency-shift keying may be accomplished by modulating the drive
current of a transmitter diode laser. The modulation may include
multi-level keying formats.
Information modulated onto carriers may be coded, such as according
to a multiple-access, error-correction, or encryption code.
Interleaving may be employed to reduce distortion effects caused by
the channel 99. The carriers may be phase-shift (or delay) coded or
otherwise coded with a multiple-access or encryption code.
Modulators may provide a modulation signal to each of the carriers,
or they may modulate a composite transmit signal formed from the
superposition of the carriers. The modulator 104 may include a
clock having a frequency that determines the modulation frequency
imparted to the data.
The modulator 104 may include a delay or phase-shift device that
delays or phase shifts one or more of the carriers before insertion
into the fiber. The delay may be applied by a timing switch or
delay device that adds a delay or phase-shift to each of the
carriers separately or combined. The delay device may consist of
one or more delay paths that provides a variable delay to the
carriers, such as a delay that depends on carrier wavelength or
polarization. The modulator 104 may provide a windowing function to
lower sidelobes. Windowing functions include spatially variant
apodization and any other methods that reduce sidelobes, such as
Hamming, Hanning, Gaussian, triangular (Bartlett), Kaiser,
Chebyshev and raised-cosine filtering.
A modulation scheme, such as pulse amplitude modulation may be
performed on the individual carriers or on the composite signal 130
shown in FIG. 3A. The modulation width may be large relative to the
pulse width. For example, the modulation width may span a plurality
of time intervals 133, 135, and 137. The modulation width may span
a short time interval such as interval 135. In either case, the
composite signal 130 received by a receiver (not shown) will be
substantially identical. Therefore, a very large modulation width
(i.e., a very slow modulation frequency) can be used to generate
high-bandwidth modulated signals.
FIG. 3A shows a plurality of phase spaces 123, 125, 127, and 129.
Phase space (which is described in PCT Appl. No. WO99/41871) is the
phase relationship between different carriers. The zero-phase
relationship 125 corresponds to a constructive interference signal
(such as a pulse) positioned at a particular instant in time. This
enables the composite signal 130 to be detectable by a receiver
(not shown) that does not adjust the phase of the individual
carrier signals. Non zero-phase relationships, such as phase spaces
123, 127, and 129, correspond to a substantially zero composite
signal 130. Windowing at either or both the transmitter 100 and the
receiver 200 may reduce the sidelobes of the composite signal
130.
Although the composite signal 130 may have substantially zero
amplitude in time intervals where there is a non zero-phase
relationship between the carriers, the carriers still exist and
therefore, the information signal represented by the constructive
interference that occurs at zero phase exists in non zero phase.
Recovery of the information signal from a non zero-phase sampling
of the carriers (such as may be required due to chromatic
dispersion in the propagation channel) may be achieved by phase
shifting (or delaying) the carrier signals in order to construct a
zero-phase relationship.
In a wireless channel, the redundantly modulated multicarrier
protocols provide substantial improvements in performance over all
other protocols. CIMA provides superior bandwidth efficiency
compared to any other protocol, and it allows a seamless conversion
from orthogonal coding to quasi-orthogonal coding. CIMA provides
substantial improvements to interference rejection and signal
degradation due to multipath. Frequency diversity in CIMA reduces
transmission-power and power-control requirements. CIMA enables
simplified transmitter and receiver designs, and it enables the
implementation of ultra-wideband CDMA by using parallel processing.
The implementation of redundantly modulated multicarrier protocols
in antenna arrays introduces new array-processing capabilities.
Frequency diversity in redundantly modulated protocols introduces
new types of spatial processing that do not require antenna
arrays.
The CIMA protocol also enables a seamless transition from
orthogonal operating conditions to quasi-orthogonal operating
conditions. A detailed discussion of the operation of a basic CIMA
system is described in "Introduction of Carrier Interference to
Spread Spectrum Multiple Access," Nassar et. al. (Proceedings of
the 1999 IEEE Emerging Technologies Symposium on Wireless
Communications and Systems, Apr. 12 13, 1999), which is hereby
incorporated by reference.
Time-division multiple access may be achieved by assigning one or
more time intervals to each transmitter. A transmission system may
include at least two transmitters generating modulated multicarrier
signals offset in time. Another type of multiple access may be
achieved by assigning one or more time-dependent phase spaces to
each transmitter. The phase spaces may be sampled in multi-user
detection processes or in other processes that can enhance the
signal quality of received signals. Multiple access may be achieved
by generating and processing spread-spectrum signals (such as CDMA)
produced by setting or adjusting (such as weighting or hopping)
characteristics of multicarrier signals. In another form of
multiple access, coded multicarrier signals are processed in the
frequency domain using a multi-user type of processing, such as
cancellation or constellation methods for separating interfering
information signals.
One aspect of the present invention includes the use of a
redundantly modulated multicarrier protocol in waveguides. An
optical-fiber communication system that uses the multicarrier
protocol is illustrated in FIG. 1. Single-mode fibers are typically
used in optical communication systems because of their
high-bandwidth capabilities. Optical fibers described herein may be
any type of optical-fiber waveguide including (but not limited to)
single mode, multimode, step-index, and quadratic-index fibers.
An optical-fiber path may include at least one amplifier to
compensate for fiber attenuation and component loss. Either
equalization or pre-emphasis may be used in the optical system to
compensate for non-uniform amplifier gain. U.S. Pat. No. 5,847,862
(which is incorporated by reference) describes a method of shaping
amplifier outputs to offset depletion of high-frequency
channels.
In an optical fiber (or any type of waveguide), differences in
carrier velocity may result from dispersion. Dispersion includes
intramodal (group velocity) dispersion, such as material and
waveguide dispersion, and intermodal dispersion, such as modal
dispersion. In the preferred embodiments, it is assumed that the
optical fiber has a dispersion that increases with increasing
signal wavelength.
FIG. 8 shows a variation in index of refraction n.sub.index with
respect to signal wavelength .lamda. in an optical fiber. The
variation of n.sub.index with respect to .lamda. results in
dispersion of broadband signals propagating in the fiber. A
dispersion profile is a dispersion-versus-wavelength (or frequency)
relationship of a dispersive medium.
FIG. 9 shows the relative position of two waves of a plurality of
waves coupled into a dispersive waveguide 99. Two waves 71A and 81A
are coupled into the waveguide with a time offset. Wave 81A has a
longer wavelength than wave 71A and therefore travels faster
through the waveguide 99. The waves 71A and 81A (as well as other
waves in the group) are substantially out of phase, resulting in a
composite signal 130A having a nearly zero amplitude. As the waves
71A and 81A travel through the waveguide, the longer-wavelength
wave 81A travels faster than wave 71A. At another location in the
waveguide 99, waves 71B and 81B are closer with respect to time
than waves 71A and 81A. The waves 71B and 81B are still
substantially out of phase, resulting in a nearly undetectable
composite signal 130B. However, the composite signal 130B has a
shorter time duration than signal 130A.
At another location in the waveguide 99, dispersion causes a group
of different-frequency waves 71C and 81C to be in phase. The
composite signal 130C resulting from the superposition of the waves
71C and 81C includes a constructive-interference pulse that is
easily detectable. The duration of the composite signal 130C is
shorter than the duration of the other composite signals 130B and
130A. The duration of the detectable portion of signal 130C may be
substantially shorter than the actual duration of the signal
130C.
Dispersion will cause the waves 71C and 81C to move out of phase at
other locations past the location where the waves 71C and 81C
combine in phase. Matching dispersion profiles and phase
relationships enables signals to be enhanced by dispersion. As
matched signal components travel through a waveguide, the duration
of composite signals is reduced and the detectability of the
composite signals is increased. The duration and detectability of a
composite signal may be optimized at one or more locations along a
waveguide.
A phase relationship applied to a multicarrier (or
frequency-diverse) signal matches a dispersion profile of a
waveguide for a specific distance if the carriers have a
predetermined phase relationship after traveling that distance
through the waveguide. The predetermined phase relationship at the
specific distance along the waveguide may be a zero-phase or a non
zero-phase relationship.
A virtual address is the phase relationship of a transmitted
multicarrier (or frequency-diverse) signal required to produce a
predetermined phase relationship at a predetermined receiver along
a waveguide. A virtual address may be represented as one or more
phase spaces, such as the phase spaces 123, 125, 127, and 129 shown
in FIG. 3A. A transmitter applies the virtual address to the
multicarrier signal. The characteristics of the address depend on
the waveguide dispersion, the length of the waveguide between the
transmitter and the receiver(s), and the characteristics of the
predetermined phase relationship of the signal(s) required at the
receiver(s).
Virtual switching includes a process of addressing a transmitted
signal such that it has a predetermined phase relationship upon
reception by at least one receiver. The addressing is a type of
dispersion compensation. The phase relationship of the addressed
signals is selected such that as the signals propagate through the
waveguide and distort due to dispersion, the phase relationships
mutate to create a predetermined phase relationship at a specific
receiver location along the waveguide. At other locations along the
waveguide, the signals may have phase relationships that cause them
to be disregarded or undetected by receivers at those
locations.
FIG. 10 illustrates the process of addressing. A transmitter 100
couples addressed multicarrier transmission signals into a
waveguide 99 that has at least two nodes 161A and 161B. The nodes
161A and 161B, which are at different locations along the waveguide
99, may include couplers (not shown) to receivers (not shown) or
other communication channels (not shown). The transmitter 100
includes a modulator 104, a multicarrier-signal generator 102, and
a coupler 150 that couples modulated multicarrier signals to the
waveguide 99.
The transmitter 100 may include an address applicator (not shown)
or the modulator 104 may perform address application to the
transmission signals. The address applicator (not shown) selects at
least one relative phase relationship between a plurality of
carrier signals having different values of at least one
orthogonalizing property (such as frequency). The relative phase
relationship corresponds to at least one address. The address
applicator (not shown) produces at least one packet of carriers
having the relative phase relationship(s). Transmission signals
having at least one virtual address arrive at one or more
predetermined nodes (such as nodes 161A and 161B) with at least one
predetermined phase relationship.
The address applicator (not shown) may use a relative phase
selector (not shown) to match a virtual address to a transmission
signal based on its intended destination(s). The address applicator
(not shown) may include a packet generator (not shown) to produce a
PAM section of a multicarrier signal having a desired phase
relationship.
In the addressing process shown in FIG. 10, the transmission
signals addressed to the first node 161A arrive at the first node
161A with a predetermined phase relationship that facilitates
reception of the transmission signals. For example, the
transmission signals may have a zero-phase relationship at node
161A. However, at node 161B, the signals addressed to node 161A are
undetectable (e.g., they have non-zero phase if there is a zero
phase space detector at node 161B) or have a phase relationship
that causes them to be discarded. Similarly, transmission signals
addressed to the second node 161B arrive at that node 161B with a
predetermined phase relationship and are undetected or removed at
node 161A.
FIG. 11A shows a multicarrier transmission system that provides
addressing to transmitted signals. An information signal s.sub.k(t)
(from an input data source 101) intended for a k.sup.th user is
split and modulated onto N carriers in a first modulation step
104A. In a second modulation step 104B, a plurality of complex
weights is applied to the modulated carriers. The complex weights
include phase shifts (or delays). In an addressing step 104C
additional weights a.sub.nk are applied to the carriers to
compensate for carrier-dependent distortion effects caused by the
channel 99 between the transmission system and an intended receiver
200.
In this case the channel provides a distortion to the n.sup.th
carrier by an amount of e.sup.i.PHI..sub.n.sup.d.sub.k. .PHI..sub.n
is a linear (with respect to distance) delay factor associated with
a carrier frequency in the channel 99 and d.sub.k is the distance
that the wave travels between the transmitter and the receiver 200.
The factor .PHI..sub.n may depend on nonlinear channel effects,
such as dispersion.
The weights a.sub.nk have values of e.sup.i.PHI..sub.n.sup.d.sub.k
to compensate for the channel distortion affecting each carrier.
Therefore the weights a.sub.nk provide a type of addressing to the
transmitted signals. The weights a.sub.nk may include windowing
weights and/or weight values that facilitate signal separation by
the receiver 200. The modulation steps 104A, 104B, and 104C may be
performed in any order and may be combined. The transmitted signals
are coupled out of the channel 99 by a receiver coupler 151 coupled
to the receiver 200.
FIG. 11B shows steps of an addressing method. Multicarrier signals
are produced in a multicarrier-signal generation step 180. The
multicarrier signals may be information modulated during this step
180. In the multicarrier-signal generation step 180, the
multicarrier signals are preferably provided with a carrier-phase
relationship that matches the dispersion profile of the channel 99
for at least one desired address. The process of providing the
carrier-phase relationship may include providing at least one
initial carrier-phase relationship between the carriers and
selecting the values of at least one carrier's orthogonalizing
properties (such as frequency and/or polarization) that affect
dispersion of the signals in the channel 99.
The multicarrier signals are assigned at least one address in a
phase-space addressing step 181. Adjustments to the carrier phases
and/or the carrier frequencies may be performed in this step 181.
The process of addressing is performed by selecting at least one
set of relative phases of the multicarrier signals. The selection
process may be performed by individually modulating a portion of
each carrier or modulating a portion of the composite signal 130.
The modulation may be any type of modulation including PAM, and it
may involve modulating the carriers or composite signal 130 with at
least one information signal. The phase-space addressing step 181
may adjust or control the carrier phases and/or the orthogonalizing
properties. A coupling step 182 couples the addressed signals into
the channel 99.
Adjusting multicarrier signal frequencies may be performed at the
transmitter 100 in order to provide a phase relationship that
matches a dispersion profile of the waveguide 99. This is done in
order to provide a specific phase relationship between the carriers
at the transmitter 100 (such as to facilitate PAM of the
superposition signal). The signal frequencies may also be adjusted
in order to adjust the phase relationship of signals received by
one or more receivers (not shown) along the waveguide 99. If the
multicarrier signals are non-uniformly spaced, then dispersion
shifting of the carriers is unlikely to generate multiple
primary-interference zones associated with a single address.
Receivers described in this specification are considered to be any
system that processes transmitted signals coupled out of (received
from) a communication channel. The processing achieves recovery of
one or more information signals modulated on the transmitted
signals. The receiver 200 may include a coupler to couple signals
from the communication channel 99. The receiver 200 may provide
decoding of encrypted, error-coded, or spread-spectrum coded
signals. Signal processing may involve use of a phase-lock loop to
track phases of received signals in order to compensate for phase
variations (such as jitter).
The receiver 200 may use discreet components or digital signal
processing methods in a CPU. The receiver 200 may include one or
more discreet components or methods including envelope detectors,
filters, decoders, level controllers, amplifiers, phase-lock loops,
de-interleavers, demodulators, mixers, windows (such as
frequency-domain or time-domain windows), analog-to-digital
converters, circulators, samplers, phase shifters, weight-and-sum
processors, delay lines, pulse-stretching processors, electrical
signal generators, signal storage processors, cancellers, and
frequency converters. The receiver 200 may change the frequency of
received signals before, during, or after processing to recover the
information signal(s). The receiver 200 may convert received
signals into electrical signals and the electrical signals may be
processed in a CPU using analog or digital signal processing.
A time-domain receiver receives and processes transmitted
time-domain signals to recover one or more information signals
modulated on the transmitted signals. The time-domain receiver may
receive a time-domain signal and apply a signal-processing method
to facilitate reception of a pulse. Many types of signal processing
may be used to stretch a received pulse. RF pulse-stretching
methods are described in U.S. Pat. No. 5,805,317, which is hereby
incorporated by reference. Time-domain receivers may include
envelope detectors, peak detectors, and the like. Time-domain
receivers may also include any type of decimation-in-frequency
system, frequency analyzer, or frequency processor to assist in
detection of the received time-domain signals.
FIG. 12 shows a general form of a receiver 200 of the invention.
FIG. 12 illustrates the function and structure of a signal
processor that extracts information signals from one or more
received multicarrier signals. Some of the processes performed by
the receiver 200 may be performed by software in a computer or
digital signal-processor chip.
A coupler 150 couples received signals from a communication channel
99 into a carrier isolator 201, such as a band-pass filter. A
band-pass filter may include one or more filters of a filter bank
(not shown). The isolated received signals may be down converted by
mixing with a local oscillator 203 signal. Down conversion of the
received signals is an optional process. The received signals may
be sampled by a sampler 214 to produce a plurality of received
information bits (or otherwise processed signals) that are
processed in a processor 212. Processed signals are passed on to a
demodulator/decoder 206 that outputs a recovered information signal
s.sub.n(t).
The carrier isolator 201 separates or isolates at least one
multicarrier signal. The carrier isolator 201 may provide
filtering, diffraction, or coding (or a combination of methods) to
achieve separation of multicarrier signals. The carrier isolator
201 may include a filter bank (not shown), which is defined as any
device that separates multicarrier signals with respect to one or
more orthogonality parameters that distinguish the multicarrier
signals.
One type of filter bank is a frequency-filter bank. A
frequency-filter bank is any device or method that performs
separation-by-frequency of a frequency-diverse signal. A filter
bank may be an array of filters or a signal-processing technique
(such as a Fourier transform) that acts on a time-domain signal to
separate it into spectral components. A filter bank may include a
set of processors that spectrally decompose a time-domain signal
into a set of frequency bins. A frequency bin represents the
frequency band of each filter in a filter bank. The filter bank may
provide weights to the bins.
A Fourier transform, as used herein is defined as any of the direct
or inverse Fourier transform methods including Fourier transforms,
Fourier series, discreet-time Fourier transforms, discreet Fourier
transforms, and polynomial transforms. Fourier transforms may be
implemented using any number of computational techniques, such as
fast Fourier transforms, and they may be supported using additional
mathematical relationships such as Laplace transforms.
A wavelength demultiplexer is a type of carrier isolator 201. Many
different types of filters may be used in a carrier isolator 201.
To separate individual carriers, the filters preferably have sharp
roll-off characteristics to minimize cross talk between channels.
The carrier isolator 201 may include wide-band filters for
separating a plurality of channels into groups of channels for FDM.
The carrier isolator 201 may also include multiple stages of
wavelength demultiplexers.
A preferred embodiment of the carrier isolator 201 includes a
monolithic optical-waveguide filter. Bandpass filters may be
interferometric (such as thin-film interference filters), resonant
cavities, or acousto-optic filters. A filter may comprise a Bragg
grating in a Mach-Zehnder interferometer. Filters may be switchable
or tunable. Another method of carrier isolation may include
filtering after converting received electromagnetic signals to
electrical signals. A star coupler with tunable filters on the
receiving ends may also be used as a carrier isolator 201 for
wavelength demultiplexing signals.
Another type of carrier isolator 201 is a decoder, which may be
implemented in the demodulator/decoder 206 or in other decoders
described in the specification. A decoder may be used to describe
either or both an information decoder and a carrier decoder. The
decoder may be a multi-stage or parallel decoder and may include at
least one correlator and/or at least one matched filter.
An information decoder decodes an encoded information signal. The
decoder may provide encryption, error-correction, or
spread-spectrum decoding (or any combination of decoding) to decode
an encoded information signal.
A carrier decoder provides decoding of an encoded multicarrier
signal in which each carrier signal may be AM, FM, ASK, PSK,
frequency-hop, time-hop, delay, time-offset, or phase-space
encoded. Encoding may include differential modulation.
A spread-spectrum decoder can be used as an information decoder or
carrier decoder. The decoder may decode an information or
multicarrier signal according to a code sequence generated by a
code generator. The decoder may include a multi-stage decoder.
Decoders may generate one or more N-point transforms. N-point
transforms include DFTs, FFTs, WTs, HTs, RTs, PTs, Inverse DFTs,
Inverse FFTs, Inverse WTs, Inverse HTs, Inverse RTs, Inverse PTs,
and any other reversible transform.
The sampler 214 may sample received signals with respect to one or
more orthogonality parameters. A time-domain sampler collects
samples during multiple time intervals. A phase-domain sampler
takes at least one sample in at least one time interval, then
adjusts the relative phases of the sampled signals to reconstruct
time-domain signals occurring in other time intervals. A
space-domain sampler receives samples from a plurality of spatially
separated locations or directions of arrival. A polarization
sampler takes samples from a plurality of samplers having different
polarization sensitivities. A frequency-domain sampler includes a
filter bank for separating received signals into a plurality of
frequency components. The information signals are removed from the
carriers and the complex-valued amplitude of the information
signals is preserved. Frequency-domain processing may include the
removal of redundant transform values. A sampler (such as sampler
214) may be used as a carrier isolator.
The processor 212 may include one or more discreet components or
signal-processing methods including envelope detectors, filters,
decoders, coders, level controllers, amplifiers, phase-lock loops,
de-interleavers, demodulators, mixers, windows (such as
frequency-domain and time-domain windows), analog-to-digital
converters, digital-to-analog converters, circulators, samplers,
phase shifters, weight-and-sum processors, delay lines,
pulse-stretching processors, signal generators, local oscillators,
signal-storage processors, cancellers, and frequency
converters.
Components of the receiver 200 shown in FIG. 12 may be arranged in
different orders. In some cases, some of the components can be
removed. For example, in its simplest form, the receiver 200 may
include the coupler 150 coupled to the processor 212 wherein the
processor 212 includes an envelope detector that outputs the
information signals s.sub.n(t).
FIG. 13 shows a receiver 200 and reception method of the invention.
A coupler 150 couples received multicarrier signals from a
communication channel 99 to a carrier isolator 201. The carrier
isolator 201 separates a plurality of the multicarrier signals into
separate carrier-signal components. The carrier-signal components
are coupled into a decoder 207 that decodes either or both carrier
and information signals. The decoded signals are coupled to a
detector 204, which may include a parallel-to-serial signal
converter 255. The signal converter 255 may include at least one
weight-and-sum processor, a combiner, a canceller, and/or a
constellation processor. The signal converter 255 outputs one or
more recovered information signals s.sub.n(t).
The detector 204 may include a multi-user detector (not shown). A
multi-user detector (not shown) receives one or more signals from a
plurality of user channels and processes those signals to estimate
their values. The detector 204 can make either hard decisions or
soft decisions. The detector 204 may perform diversity combining,
which can consist of co-phasing, selective combining, maximal-ratio
combining, equal-gain combining, maximal-selection combining, or
any other type of diversity combining.
Any of the signal-processing operations associated with the
processor 212 may be incorporated into the carrier isolator 201,
the decoder 207, the detector 204, or the parallel-to-serial signal
converter 255. The decoder 207, detector 204, and
parallel-to-serial signal converter 255 may comprise one of a
plurality of sets of receivers coupled to the carrier isolator 201.
The carrier isolator 201 may separate received multicarrier signals
into a plurality of groups. Each of the carrier groups may be
coupled to a different receiver, such as receiver 200.
There are a large variety of receivers that can be used in a
redundantly modulated multicarrier waveguide-communication system
including the following: 1. An ordinary time-domain receiver that
receives carriers having a zero-phase relationship. 2. A receiver
coupled to a transmitter that retransmits the received signals into
the same channel medium. 3. A transport-medium interface: (such as
an optical-to-RF converter) for retransmitting received signals
into a different channel. 4. An optical-to-electrical converter
(which may include a digital signal processor for processing
electrical signals) and/or detector. 5. A multi-user detector or
multi-channel detector. 6. An address adjuster that uses
phase-domain sampling to produce a predefined phase relationship
between multicarrier signals. The address adjuster may apply phase
adjustments to compensate for non zero phase space signals.
Combinations of these receiver designs may be used in any waveguide
or wireless receiver.
A receiver for an optical system, such as an optical-fiber
communication system, is shown in FIG. 14A. A coupler 150 couples
signals from a communication channel 99 (such as an optical fiber
or free space) into an optical receiver 200. Couplers (such as
coupler 150) may include one or more signal-processing systems (not
shown) for filtering, shaping, or otherwise processing received
signals before detection. Received signals may optionally be down
converted by a down converter 205. A down converter, such as the
down converter 205, may be a heterodyne or a homodyne device. The
down converter 205 includes at least one local oscillator 203 and
at least one mixer 202 for mixing one or more reference signals
(generated by the local oscillator 203) with at least one of the
signals received from the channel 99 to generate one or more
down-converted signals.
A detector 204 may receive signals directly coupled from the
communication channel 99. The detector 204 may receive
down-converted signals from the down converter 205. In an optical
version of the detector 204, optical signals are converted to
electrical signals by one or more photodetectors having high
quantum efficiency in the relevant spectral range of the received
or down converted signals. The detector 204 may be an electrical or
RF detector that is responsive to baseband or
intermediate-frequency signals output from the down converter 205.
Detectors (such as the detector 204) may include signal processing
systems (not shown), such as phase-lock loops, digital signal
processors, filters, phase-shifters, amplitude adjusters,
multi-user detectors, feedback loops, synchronizers, pilot-signal
processors, combiners, and the like. Signals output from the
detector 204 may optionally be demodulated and/or decoded by a
demodulator/decoder 206.
A detector of the present invention for a multicarrier
optical-fiber system may have one of the following designs: 1. A
single photodetector sensitive to all received wavelengths. 2. A
wavelength demultiplexer to separate wavelengths, a
received-carrier adjuster (phase or amplitude), and a
photodetector. 3. A wavelength demultiplexer and a plurality of
photodetectors. Electrical outputs of the photodetectors may be
processed and combined.
FIG. 14B shows a coupler 150 for coupling signals from a
communication channel 99 to a carrier separator 201 (such as a
wavelength demultiplexer). The carrier separator 201 separates
received signals into a plurality of carrier-signal components that
are coupled to at least one receiver, such as receiver 200. A
carrier separator (such as the carrier separator 201) may separate
the received signals into separate carrier signals or groups of
carrier signals. A down converter 205 may optionally down convert
the signal components. Similarly, an up converter (not shown) may
up convert received signals or signal components. Signal up
conversion or down conversion may optionally be performed before
carrier separation.
The down converter 205 includes a local oscillator 203 and a
combiner 202. A detector 204 receives the carrier signals (which
may be down-converted or up-converted carrier signals). The
detected signals may be subjected to further processing by a
demodulator/decoder 206 that performs either or both demodulation
and decoding of the detected signals.
Demultiplexing systems (as well as multiplexers) may include
diffraction gratings and multi-layer interference filters.
Mach-Zender or Fabry-Perot interferometers may be used for
filtering the desired channel. The demodulator/decoder 206 may
include a square-law demodulator to demodulate received ASK
signals. The receiver 200 may include a phase-diversity
receiver.
FIG. 14C shows a receiver 200 having a signal-component adjuster
209. Received electromagnetic or electrical signals are coupled
into a carrier separator 201, which separates the received signals
into individual carrier signals or groups of carriers. The carriers
are coupled to the component adjuster 209 that may adjust
carrier-signal parameters (such as magnitude, phase, frequency,
polarization, and code). The adjusted carriers are coupled into a
demodulator/decoder 206 that may provide preprocessing (including
combining) before being demodulated and or decoded.
FIG. 14D shows a multi-channel (or multi-user) receiver 200 coupled
to a communication channel 99. Received signals are coupled into
the receiver 200 by a coupler 150. A coupler (such as coupler 150)
may include more than one coupling device coupled to a
communication channel 99. Received signals may optionally be down
converted by a down converter 205. A splitter 210 splits
down-converted or directly coupled signals. The splitter 210 may be
a carrier separator (not shown) or the splitter 210 may produce
multiple versions of the received signals by duplicating, sampling,
or splitting the received signals. The split signals are coupled
into a plurality of preprocessors, such as preprocessors 211A and
211B.
The receiver 200 may receive multiple transmissions from at least
one transmitter. The received signals are preferably separable
through a multiple-access technique based on at least one diversity
parameter, such as spread-spectrum code, frequency, time,
differential power, polarization, or phase space.
The preprocessors 211A and 211B may include an address separator to
demultiplex received signals with respect to at least one diversity
parameter. If at least one of the diversity parameters is phase
space, the preprocessors 211A and 211B may include one or more
phase processors (not shown) for decoding multicarrier signals
having different phase-spaces. A phase processor (not shown)
applies phase adjustments to a plurality of carrier signals to
provide at least one predetermined phase relation. A phase
processor (not shown) may include a plurality of phase processors
(not shown) for applying a plurality of phase adjustments to
compensate for a plurality of different phase relationships. The
preprocessors 211A and 211B may provide other digital signal
processing techniques to the signals, such as filtering, phase
adjustment, phase stabilization, amplitude adjustment, and
decoding.
The separated signals may be demodulated and/or decoded.
Interference in the signals may be removed by an optional
interference canceller 256. The receiver 200 may include at least
one phase-locked loop.
An alternative embodiment of a receiver 200 that performs address
separation is shown in FIG. 14E. A coupler 150 couples optical
carrier signals from a communication channel 99 into a first
preprocessor 211A that applies phase adjustments to a plurality of
the carrier signals. Outputs from the preprocessor 211A are coupled
into a nonlinear transmission medium (e.g., a KDP crystal) 215A.
The transmission medium 215A is coupled to a second preprocessor
211B that applies phase adjustments to a plurality of the carrier
signals. Outputs from the preprocessor 211B are coupled into a
second nonlinear transmission medium 215B. Additional
preprocessor/transmission-medium systems may be arranged in
series.
A reference source 217 produces a plurality of reference beams that
are coupled into each nonlinear transmission medium 215A to 215B. A
nonlinear process (such as second-harmonic generation) may be used
to generate an information signal resulting from the interaction of
the multicarrier signals and the reference beams. Other techniques
for generating an information signal may be used instead, such as a
threshold-power detection technique in which signals output from
the preprocessors may excite a gain medium if the carriers are in
phase. A detector, such as detectors 204A and 204B receives each of
the information signals. The detected signals may be demodulated,
decoded, and/or acted upon by an interference canceller (not
shown).
FIG. 15 shows a transport-medium interface that couples received
optical signals from an optical waveguide 99A to a free-space
propagation environment 99B. A first coupler 150 couples optical
signals out of the waveguide 99A. The optical signals are
optionally down converted by a down converter 205. The down
converter 205 may provide a plurality of mixing signals to the
optical signals. The optical signals may be separated according to
one or more diversity parameters (such as wavelength) before being
down converted. Down conversion may include reducing the frequency
separation of the optical signals. The down-converted signals are
coupled into the free-space propagation environment 99B by a second
coupler 151. The coupler 151 may be an antenna system (not shown)
including a single antenna element or an array of antennas. The
coupler 151 may include at least one processor (not shown) to
control beam forming and directionality.
The down converter 205 may include any type of optical-to-RF
converter (not shown). An optical-to-RF converter is a device or
method that down converts an electromagnetic signal to a signal
having a lower frequency. This includes mixers, optical-heterodyne,
and optical-homodyne devices. The down converter 205 may include a
homodyne device if the optical carriers are modulated with RF
signals. The down converter 205 may be any device that has an input
of at least one information signal modulated on at least one
optical carrier and that outputs at least one RF carrier that is
modulated with the information signal(s). The down converter 205
may perform an intermediate process of converting an input
electromagnetic signal into an electrical signal, which can be used
to modulate one or more RF output signals. A processor (not shown)
may perform one or more signal processing steps on an input signal,
such as filtering, amplifying, windowing, phase shifting, encoding,
decoding, storing, duplicating, inverting, and weighting.
Redundantly modulated multicarrier signals may be used as a
multiple-access communication protocol such as CIMA, MC-CDMA, or an
OFDM protocol that transmits data over multiple carriers. CIMA
signals have advantageous transmission characteristics in a
wireless environment. CIMA signals can be used to construct many
different wireless protocols including GSM, other TDMA protocols,
and CDMA. CIMA provides substantial improvements to system
capacity, simplicity, and signal quality, and it greatly increases
diversity benefits of conventional multiple-access protocols. CIMA
also enables a simple transport-medium interface between
optical-fiber and wireless transmissions because a wireless
protocol constructed from multiple carriers does not require a
protocol change at the interface.
The transport-medium interface may be designed to couple received
signals from the free-space channel 99B to the waveguide 99A. The
transport-medium interface may include a RF-to-optical converter
(not shown) for converting received wireless RF signals into
optical signals that are inserted into the waveguide 99A.
FIG. 16 outlines a process of transmitting signals that transition
between a waveguide and a wireless channel without requiring a
change in protocol. A transmission step 281 involves transmitting
at least one CDMA-type CIMA signal into an optical waveguide. A
waveguide-propagation step 282 provides propagation of the
transmitted signal(s) through the waveguide. Signals are coupled
out of the waveguide and into a wireless channel in a
transport-medium interface step 283. This step 283 may involve
changing the wavelength of the signals by a frequency-conversion
method, such as optical-heterodyne or optical-homodyne processes. A
free-space propagation step 284 describes the propagation of the
signals coupled into the wireless channel. The free-space
propagating signals are received in a wireless-signal reception
step 285.
FIG. 17A shows a waveguide communication system that includes a
cancellation line 199 to cancel signals received at a desired
receiver 200A after they propagate past the receiver 200A. A first
node 150A receives a desired signal from a waveguide-communication
channel 99. The node 150A includes a transmitter 100A and the
receiver 200A. The signal received at node 150A continues to
propagate through the channel 99 to other nodes, such as a node
150B. Node 150B includes a transmitter 100B and a receiver
200B.
If the signal is a multicarrier signal having a phase relationship
that matches the phase profile of the waveguide 99, then the signal
may be undetectable at node 150B. However, if the carriers are
uniformly spaced in frequency, then a detectable
constructive-interference signal will be detectable at more than
one location in the waveguide 99. The carrier signals may have a
mode relationship that causes a plurality of detectable
constructive-interference signals to occur at multiple locations in
the waveguide 99.
At least one cancellation channel 199 is coupled between node 150A
and 150B. The cancellation channel 199 couples the receiver 200A to
the transmitter 100B. A desired signal received at node 150A may be
coupled from the receiver 200A to the transmitter 100B. Thus, node
150B receives two versions of node 150A's desired signal. One
version is the wave that propagates through the communication
channel 99 to provide a channel-shifted version of node 150A's
desired signal. The second version is received from the
cancellation channel 199 and inserted into the node 150B by the
transmitter 100B. Preferably, the second version is a cancellation
signal that is an inverse or out-of-phase replica of the
channel-shifted version. Canceling node 150A's desired signal at a
later stage in the waveguide 99 network enables reuse of node
150A's address space in other parts of the network.
The cancellation channel 199 may be a waveguide or wireless
channel. A received signal at node 150A may be inverted or
otherwise adjusted by either the receiver 200A or the transmitter
100B. The channel 199 may also provide signal processing (such as
phase adjustment that naturally results from dispersion in a
nonlinear medium). The channel paths of both the communication
channel 99 and the cancellation channel 199 may each be oriented to
provide an equal amount of delay to the signals received by
receiver 200A.
The cancellation-channel 199 may include a separate waveguide or it
may include a wireless channel. The cancellation channel 199 may be
represented by one or more signals that have higher velocities than
the signal(s) in the communication channel 99. The
cancellation-channel 199 may consist of signals having a
predetermined polarization, mode, and/or wavelength. Although not
shown, one or more cancellation channels may connect nodes 150A and
150B to other nodes (not shown).
FIG. 17B shows a waveguide communication system that cancels
signals received at a desired receiver 200A after they propagate
past the receiver 200A. The communication system shown in FIG. 17B
has similar components to the system shown in FIG. 17A except that
each receiver-transmitter pair at each node 150A and 150B has a
receiver-transmitter coupling, and the cancellation channel 199
(shown in FIG. 17A) is replaced by a virtual cancellation channel
(not shown). The virtual channel is defined by a signal path of a
cancellation signal inserted into the communication channel 99. The
cancellation signal has at least one different signal
characteristic (such as polarization, mode, or wavelength) that
enables it to propagate faster than the desired communication
signal received by receiver 200A.
The receiver 200A receives the desired signal and processes it to
create a cancellation signal having at least one different signal
characteristic (such as polarization, mode, or wavelength). Other
processing steps (such as, but not limited to, phase shifting,
delay, amplitude adjustment, and filtering) may be performed by the
receiver 200A and/or the transmitter 10A. The transmitter 100A
transmits the cancellation signal into the channel 99. The
cancellation signal may be coupled into the channel 99 at a
different node than node 150A.
The receiver 200B preferably receives the cancellation signal
before it receives receiver 200A's desired signal. The cancellation
signal is processed by either or both the receiver 200B and the
transmitter 100B to ensure that the cancellation signal will cancel
receiver 200A's desired signal. This processing may include steps
to return the cancellation signal to the same frequency, mode,
and/or polarization as receiver 200A's desired signal received at
the node 150B or any other node (not shown) where the cancellation
signal may be inserted.
A process for canceling communication signals from in a
communication channel is shown in FIG. 17C. Signals are coupled out
of a communication channel in a coupling step 291. At least one
desired signal is detected from the coupled signals in a separation
step 292. The desired signals are coupled into a cancellation
channel that runs parallel to the communication channel in step
293. The signals from the cancellation channel are coupled into the
communication channel to cancel the signals received by the
receiver in a final step 294. Adjustment of the desired signal to
create a cancellation signal may be performed in either or both
steps 293 and 294.
Redundantly modulated multicarrier signals, such as CDMA-CIMA
signals, enable signal differentiation in both time and frequency
domains. CDMA-CIMA codes that are unique in the time domain also
have unique frequency-versus-amplitude profiles. The DS-CDMA
signals are determined by weights applied to each carrier.
Therefore, frequency diversity as well as code diversity can be
used to achieve multiple access.
FIG. 18 shows a process diagram for a diversity-based cancellation
system. Each of a number N of information signals s.sub.n(t)
receive a plurality of weights in a weighting process 440. A
weighting system (not shown) may apply real or complex weights to
the signals s.sub.n(t). The weighting system may include magnitude
and phase adjustment circuits or processes and the weights may be
deterministic or adaptive.
The weighted signals are processed in a coding process 444. The
coding process 444 may include the weighting process 440. The
coding process 444 may include one or more N-point transforms. The
variable N in an N-point transform does not necessarily correspond
to the number N of information signals or other variables used
throughout this specification. N-point transforms include DFT, FFT,
WT, HT, RT, PT, Inverse DFTs, Inverse FFTs, Inverse WTs, Inverse
HTs, Inverse RTs, Inverse PTs, and any other reversible transform.
The coding process 444 can be regarded as a multicarrier-generation
process. M coded signals are coupled into a communication channel
99, which may operate on the signals. Coded signals coupled out of
the channel 99 are decoded in a decoding process 244. The decoding
process 244 may include at least one M-point transform. A decoding
process, such as decoding process 244, can be regarded as a
multicarrier-separation process. A separator (not shown) may
perform the decoding process 244.
A set of M decoded signals are coupled into an
interference-cancellation process 250 that separates the desired
signals s.sub.n(t) from interference and outputs the separated
desired signals s.sub.n(t). The interference-cancellation process
250 may include cancellation and/or constellation processes.
Throughout this specification, the term "interference" is meant to
convey any interfering signals including other desired signals
s.sub.n(t). The interference-cancellation process 250 or the
decoding process 244 may separate the modulated desired signals
s.sub.n(t) from the carrier signals, or reduce the received
carriers to a carrier having a common diversity parameter. The
interference-cancellation process 250 is a signal-separation
process. Any type of signal-separation process may be used. The
interference-cancellation process 250 may include any type of
multi-user or multi-channel detection processes.
The diversity-based cancellation process shown in FIG. 18 is
described in U.S. patent application Ser. No. 09/347,182 wherein
the coding process 444 is an inverse FFT and the decoding process
244 is an FFT. The decoding process 244 produces M equations with N
unknown signals s.sub.n(t). The M equations result from detection
(or separation) of the M carriers. To explicitly solve for the N
unknowns, M must be greater or equal to N.
The interference-cancellation process 250 may perform signal
analysis using a different diversity parameter than the one or more
diversity parameters that define the carriers. For example,
frequency-diverse carriers may be summed and evaluated in the
time-domain to separate information signals s.sub.n(t) encoded on
the carriers. Weight-and-sum processes (or other types of
cancellation) may be performed on the time-domain signals in order
to remove interference and separate the desired signals
s.sub.n(t).
FIG. 19 shows a time-domain representation of a plurality of
frequency-domain encoded signals. Each pulse 1, 2, 3, 4, and 5
represents a CIMA signal having three (M=3) multi-frequency carrier
signals. CIMA enables 2M quasi-orthogonal signals in the time
domain if data symbols modulated on the carrier signals are limited
to binary phase shift key or amplitude modulation, whereas carrier
processing yields only M equations. Since the quality of
quasi-orthogonal signals can be improved using multi-user detection
(which involves the same processes as interference cancellation),
processing signals in a diversity-parameter domain that enables
quasi-orthogonality of the signals being processed increases the
capacity of the communication system. This realization may be
extended to many different diversity-parameter domains. For
example, many types of multicarrier-defined diversity-parameter
domains (such as frequency) may be used to generate CIMA signals
that can be processed in the time domain. One of the benefits of
alternative diversity-parameter processing is that, in some cases,
the benefits of both diversity and enhanced capacity can be
obtained.
Different diversity parameters may be combined to increase capacity
and/or diversity benefits in a communication system. FIG. 20A shows
a process diagram of a multiple-diversity communication system. A
plurality N of information signals s.sub.n(t) are weighted in a
plurality N of weighting processes 440.1 to 440.N. Weighted
information signals are coded by a plurality L of coders 444.1 to
444.L or the weighted information signals control the coding of
carrier signals generated by the coders 444.1 to 444.L. The coded
signals are coupled into a communication channel 99.
Coded signals are coupled out of the communication channel 99 and
decoded by a plurality of decoding processes 244.1 to 244.P. The
decoded signals are coupled to a plurality of signal-separator
processes 256.1 to 256.P. Each of the signal-separator processes
256.1 to 256.P generates a plurality of signals representing
equations having a number N' of unknowns. A plurality of the
signal-separator processes 256.1 to 256.P generates a number of
equations that does not equal or exceed the number N of unknowns.
However, the number of equations generated by all of the
signal-separator processes 256.1 to 256.P equals or exceeds the
number N of unknowns. A second-stage signal-separator process 257
may be implemented to combine the equations generated by the
signal-separator processes 256.1 to 256.P and determine explicitly
the values of the information signals s.sub.n(t). A
signal-separator process (such as the signal-separator processes
256.1 to 256.P) may be any interference-cancellation process (using
either or both cancellation and constellation processes), such as
multi-user detection or multi-channel detection.
One or more of the signal-separator processes 256.1 to 256.P and
257 may perform quasi-orthogonal signal separation in an
alternative diversity-parameter domain. For example, one or more of
the signal-separator processes 256.1 to 256.P and 257 may include
combining received signals and processing the superposition of the
signals in the time domain.
FIG. 20B shows an alternative process diagram for a
multiple-diversity communication system. A plurality N of
information signals s.sub.n(t) are weighted in a plurality N of
weighting processes 440.1 to 440.N. Weighted information signals
are coded by a plurality L of coders 444.1 to 444.L or the weighted
information signals control the coding of carrier signals generated
by the coders 444.1 to 444.L. The coded signals are coupled into a
communication channel 99.
Coded signals are coupled out of the communication channel 99 and
decoded by a plurality of decoding processes 244.1 to 244.P. The
decoded signals from each decoding process are coupled into a
plurality of signal-separator processes 256.1 to 256.P. Outputs
from the signal-separator processes 256.1 to 256.P may be coupled
to a second-stage signal-separator process 257.
Signal coding (described throughout the specification) with respect
to transmitter-side coding can involve encoding carrier signals
with weights according to a signal profile (such as a spatial gain
distribution at a receiver) that facilitates the separation of
multiple received information signals having the same carriers. The
weights may be deterministic or adaptive. The weights may be
complex. A weight coder may include at least one phase shifter
and/or delay device. Decoding processes may include cancellation or
constellation methods of signal estimation.
U.S. patent application Ser. No. 08/862,859 describes spatial gain
variations of signals transmitted in free space. The spatial gain
of a received signal is the complex-valued amplitude of the signal
relative to at least one signal space. A signal space is defined by
at least one diversity parameter. PCT Appl. No. WO95/03686,
entitled "Active Electromagnetic Shielding" describes how receivers
that are spatially separated receive different proportions of
signals from spatially separated transmitters. The Active
Electromagnetic Shielding application also describes cancellation
circuits that can be used to separate desired signals from
interfering signals.
Spatial gain distributions describe all effects that result in the
complex amplitude or other characteristic of a signal varying with
respect to a diversity parameter (or signal space). Spatial gain
distributions result from propagation effects including, but not
limited to multipath fading, shadowing, absorption, scattering,
path loss, and diffraction. Spatial gain distributions may also be
determined by either or both transmitter- and receiver-related
parameters, such as directivity, masking, diffraction,
polarization, phase space, and coding.
Both wireless and guided-wave signals have spatial gain variations.
In free space, spatial gain variations result from many
environmental effects such as shadowing, multipath, absorption,
scattering, and path loss. Spatial gain variations can also be
affected by the transmission system, which can control beam shape,
directionality, and polarization. Dispersion, reflections,
attenuation, and amplification can affect spatial gain in a
waveguide.
Frequency gain variations can result from the frequency-dependent
nature of spatial gain variations. Frequency gain is the complex
amplitude-versus-frequency distribution of a frequency-diverse
signal. Differences in the amplitudes of each frequency component
of frequency-diverse signals transmitted from different
transmitters enable multiple access via cancellation or
constellation processing methods. U.S. patent application Ser. No.
09/347,182 describes the use of frequency diversity as a spatial
processing technique that does not require an antenna array.
Another benefit of the frequency-diversity method compared to
spatial diversity methods is that it does not rely on the
fast-fading environment of the communication channel. Frequency
diversity multiplexing can be performed in any multipath
environment.
FIG. 21A illustrates two diversity parameters that can be used for
increasing diversity or capacity. Spatial processing is illustrated
by a horizontal bar 21. Spatial processing is performed in a
particular frequency band (such as f.sub.1) in order to enable
reuse of that band. System capacity can be proportional to the
number of spatial locations (such as S.sub.1, S.sub.2, and S.sub.3)
from which signals are sampled. Optimal bandwidth efficiency is
achieved when each frequency band is used to transmit different
information streams. In conventional multiple-access schemes,
redundantly transmitting information in the frequency domain
reduces bandwidth efficiency.
Frequency-diversity processing is illustrated by a vertical bar 22
that spans multiple frequencies f.sub.1, f.sub.2, and f.sub.3.
Information is redundantly transmitted on each of the frequency
bands. Using the cancellation or constellation methods described in
the '182 application, multiple redundantly transmitted information
signals can be separated. Frequency-diversity processing may be
performed at multiple receiver locations (such as S.sub.1, S.sub.2,
and S.sub.3) in order to achieve the optimal bandwidth
efficiency.
Frequency-diversity processing benefits systems that have a small
number of antennas by providing frequency reuse and mitigating
signal loss due to deep fades. Although it is counter-intuitive to
redundantly modulate carrier signals when attempting to increase
capacity, redundant-modulation techniques (such as
frequency-diversity processing and CIMA) can provide improved
capacity as well as diversity. A unique aspect of the invention is
that redundant modulation with respect to a diversity parameter
achieves an increase in bandwidth efficiency.
FIG. 21B illustrates the use of nested interferometry to optimize
capacity. Redundantly modulated carriers having different
frequencies are frequency-diversity processed at each spatially
separated receiver. Each receiver produces three equations having a
number of unknown information signals. These equations can be
linked together via spatial processing (which in this case is the
process of combining the equations) to create nine unique
equations. Therefore, up to nine different unknown signals can be
solved explicitly in this example.
Either of the diversity parameters shown in FIG. 21A and FIG. 21B
may be replaced by another diversity parameter. Signal-diversity
parameters include signal characteristics, such as polarization
states, frequencies, time, phase space, modes, directionality, and
CDMA or other spread-spectrum codes. Carrier signals may be defined
by any of these parameters or any combination of these parameters.
FIG. 21A and FIG. 21B may include additional axes representing
additional diversity-parameter dimensions that may be used for
multiplexing, diversity, and interferometric combining.
FIG. 22A shows a frequency-diversity interferometry multiplexing
system. A plurality of transmitters 100.1 to 100.M each includes a
carrier-signal generator 102.1 to 102.M, a carrier-code generator
106.1 to 106.M, an information-signal modulator 104.1 to 104.M, and
a coupler 150.1 to 150.M coupled to a communication channel 99. At
least one receiver 200 is coupled to the communication channel 99.
The receiver 200 includes a separator, such as wavelength
demultiplexer 201, which is coupled to a down converter 205 and an
interference canceller 256. A down converter (such as down
converter 205) may be a decoder.
Each carrier-signal generator 102.1 to 102.M generates a plurality
of carrier signals. Each of the carrier signals is distinguished by
values of one or more diversity parameters. In this case, the
carrier signals are distinguished by different frequencies
(f.sub.1, f.sub.2, . . . f.sub.n). It is assumed that each of the
signal generators 102.1 to 102.M generates a similar set of carrier
signals. Each set of carrier signals is modulated by a plurality of
carrier codes (c.sub.mn) from its respective carrier-code generator
106.1 to 106.M. The codes generated by the carrier-code generator
106.1 to 106.M appear unique when observed by the receiver 200.
Each set of coded carrier signals is modulated by one of the
information-signal modulators 104.1 to 104.M. Each of the modulated
carriers is coupled into the communication channel 99. The channel
99 may be wireless, waveguide, or a combination of both.
Transmitted signals are coupled out of the communication channel 99
and received by the receiver 200. The receiver 200 demultiplexes
the received signals into wavelength (or frequency) components.
Wavelength demultiplexing may include converting the received
signals to electrical signals and performing digital signal
processes, such as Fourier transforms. Demultiplexing may also be
performed using conventional optical demultiplexing techniques. The
demultiplexed signals are down converted to a common frequency
band. The frequency band may be the baseband information signal or
some intermediate frequency. The down converting process may be
performed during wavelength demultiplexing. The down converted
signals are coupled into a canceller 256, which separates the
interfering signals using a cancellation method, such as weight and
sum. The canceller 256 may also perform a constellation method in
addition to, or instead of the cancellation method.
Separation (i.e., an explicit solution) of the information signals
depends on receiving a number of algebraically unique proportions
of the signals by the receiver. Separation quality (e.g., signal to
noise, signal to interference, or signal to noise plus
interference) depends on the proportions of the received signals.
The proportions are determined by the carrier codes applied to
carrier signals and the effect of the channel on the transmitted
carriers.
Optimizing the separation quality of the received signals can be
achieved by adjusting the carrier codes and the channel
characteristics. Carrier codes are adjusted by any of the
carrier-code generators 106.1 to 106.M. The channel 99 can be
adjusted by adjusting transmission characteristics that affect the
channel 99. In a wireless system, the directionality of a
transmitting antenna determines the channel through which
transmitted signals propagate. In either of these cases, a known
training sequence may be used to optimize the separation quality.
The training sequence may be performed in a predetermined
orthogonal channel, such as a time interval, spread-spectrum code,
frequency band, directivity, phase space, or polarization.
FIG. 22B shows a cascaded interferometry system that uses
frequency-diversity interferometry multiplexing and spatial
interferometry multiplexing. A cascaded interferometry system is
defined as a communication system that performs interferometry with
at least two different diversity parameters to enhance capacity.
The transmitters are the same as in FIG. 22A. However, there is a
plurality of spatially separated receivers 200.1 to 200.M. Each of
the receivers 200.1 to 200.M includes a separator, such as a
wavelength demultiplexer 201.1 to 201.M, coupled to a down
converter 205.1 to 205.M. The outputs of the down converters 205.1
to 205.M are input to an interference canceller 256.
Coded transmission signals that are coupled into the channel 99
have an amplitude-versus-frequency profile that depends on the
coding of the carrier signals. As the signals propagate through the
channel 99, their amplitude-versus-frequency profile can change.
Signals may exhibit different amplitude-versus-frequency profiles
at different locations in the channel 99. Signals in the channel 99
are expressed by the following equation:
C.sub.1(x)s.sub.1(t)+C.sub.2(x)s.sub.2(t)+ . . .
+C.sub.N(x)s.sub.N(t) C.sub.n(x) is the amplitude-versus-frequency
profile associated with the n.sup.th transmitted information signal
s.sub.n(t). The value of the amplitude-versus-frequency profile
C.sub.n(x) depends on the n.sup.th code applied to the signal
s.sub.n(t) and a channel parameter x. The channel parameter x
describes the state of the communication channel at a specific
location in the channel relative to the location of the
transmitter(s). Signals R.sub.k(t) received by a k.sup.th receiver
are given by the following equation:
R.sub.k(t)=C.sub.1(x.sub.k)s.sub.1(t)+C.sub.2(x.sub.k)s.sub.2(t)+ .
. . +C.sub.N(x.sub.k)s.sub.N(t) The amplitude-versus-frequency
profile C.sub.n(x.sub.k) of signals received by the k.sup.th
receiver may depend on the relative location (and in some cases,
the absolute location) of the k.sup.th receiver with respect to the
transmitter(s).
The received signals R.sub.k(t) are wavelength demultiplexed (e.g.,
separated into their component wavelengths or frequencies) into M
component signals. The information signals s.sub.n(t) are removed
from the component signals or otherwise converted to signals having
a common carrier frequency. The demultiplexing and down-conversion
processes produce a plurality M of component signals R.sub.km(t)
representing combinations of the information signals s.sub.n(t).
The component signals R.sub.km(t) may represent either linear or
nonlinear combinations of the information signals s.sub.n(t).
Preferably, the combinations are algebraically unique.
An expression for a particular component signal R.sub.km(t) that
consists of a linear combination of information signals s.sub.n(t)
is represented by: R.sub.km(t)=(.alpha..sub.1k+.alpha..sub.2k+ . .
. +.alpha..sub.Nk)s.sub.1(t)+(.beta..sub.1k+.beta..sub.2k+ . . .
+.beta..sub.Nk)s.sub.2(t)+ . . . +(.zeta..sub.1k+.zeta..sub.2k+ . .
. +.zeta..sub.Nk)s.sub.N(t) Each of the information signals
s.sub.n(t) has a series of scaling factors .alpha..sub.mk,
.beta..sub.mk, . . . , .zeta..sub.mk that depends on the
amplitude-versus-frequency profile C.sub.n(x) applied to the
carrier signals. The values of the scaling factors also depend on
the effect of the communication channel 99 on the profile. The
number N of scaling factors in each series is the number of signals
s.sub.n(t) transmitted by different transmitters. Because there are
M component signals R.sub.km(t) (which represent M equations of N
unknowns), it is preferable that M be greater or equal to the
number N of unknowns if there is only one receiver.
If there are K receivers, the number of component signals
(equations) R.sub.km(t) presented to the canceller 256 is
K.cndot.M. If the number of algebraically unique equations input to
the canceller exceeds the number of unknowns (information signals
s.sub.n(t)), the unknowns can be solved explicitly. The output of
the canceller 256 includes the information signals s.sub.n(t) or
estimates of the information signals s.sub.n(t).
FIG. 23 shows a spread-spectrum interferometer that can be used in
a communication channel 99. At least two transmitters 100A and 100B
transmit redundantly coded information signals s.sub.n(t) that are
received and decoded by at least one receiver 200. The received
signals undergo interference cancellation to separate or estimate
the information signals s.sub.n(t).
A first transmitter 100A includes a signal modulator 104A that
receives at least one information signal s.sub.1(t) and provides a
plurality of weights .alpha..sub.1 and .alpha..sub.2 to the
information signal s.sub.n(t) to generate a plurality of weighted
information signals. The weighted information signals may be used
to modulate a plurality of spread-spectrum signals produced by a
multicarrier-signal generator 102A wherein each of the
spread-spectrum signals is considered a carrier. The
spread-spectrum signals may be CDMA, Frequency Hopped, Time Hopped,
hybrid spread spectrum, N-point transform, or any type of
multicarrier spread-spectrum signals. The weighted information
signals may be input to the multicarrier-signal generator 102A and
processed to produce a plurality of spread-spectrum signals that
are information coded. The information-coded signals are coupled
into the communication channel 99 by a coupler 150A. In this case,
the communication channel 99 is a wireless channel and the coupler
includes an antenna 158A. The signals that are coupled into the
channel 99 by the first transmitter 100A are represented by the
following expression: C.sub.1(.alpha..sub.1s.sub.1(t))+C.sub.2
(.alpha..sub.2s.sub.1(t))
A second transmitter 100B that has the same general design as the
first transmitter 100A couples a plurality of spread-spectrum
carrier signals into the channel 99. Each spread-spectrum carrier
signal is modulated with at least one weighted (.beta..sub.1,
.beta..sub.2) information signal s.sub.2(t). The signals that are
coupled into the channel 99 by the second transmitter 100B are
represented by the following expression:
C.sub.1(.beta..sub.1s.sub.1(t))+C.sub.2
(.beta..sub.2s.sub.2(t))
Spread-spectrum signals C.sub.1 and C.sub.2 represent different
coded spread-spectrum signals. The spread-spectrum signals have
characteristics that depend on their coding and the signals that
they encode. Although two or more spread-spectrum signals (such as
C.sub.1(.alpha..sub.1s.sub.1(t)) and
C.sub.1(.beta..sub.1s.sub.1(t)) use the same code, the coded
signals have values that depend on their arguments
(.alpha..sub.1s.sub.1(t) and .beta..sub.1s.sub.1(t)). A coupler
150C that includes at least one antenna 158C couples the
transmitted signals out of the channel 99 for providing received
signals to the receiver 200.
The values of the coded signals are realized upon decoding the
spread-spectrum signals C.sub.1(.alpha..sub.1s.sub.1(t)) and
C.sub.1(.beta..sub.1s.sub.2(t)) and separating interfering signals.
A decoder 222 decodes the received signals using a plurality of
inverse spreading codes. If multiple information signals had been
encoded with the same spread-spectrum code, the process of decoding
those signals produces multiple interfering information signals.
The interfering signals are input to an interference canceller 256
that separates the signals using cancellation or constellation
techniques.
The values .alpha..sub.1, .alpha..sub.2, .beta..sub.1, and
.beta..sub.2 applied to the transmitted information signals
s.sub.1(t) and s.sub.2(t) represent any method of adjusting the
information signals s.sub.1(t) and s.sub.2(t) to allow
differentiation between decoded received signals. The step of
adjusting the information signals s.sub.1(t) and s.sub.2(t) may
result from the signals propagating in the channel 99.
Differentiation may be achieved by any combination of interference
cancellation, constellation techniques, filtering, and
demodulation.
FIG. 24 shows a redundant-carrier communication system in which a
plurality of carriers are received and separated with respect to at
least one diversity parameter and then processed and combined with
respect to another diversity parameter. In particular, carriers
that are defined by signal frequency are modulated with
time-dependent or phase-dependent coded information signals. A
signal consisting of the modulated carriers is received and
separated into individual modulated carriers. The carriers are
decoded and summed to recover the time-domain information
signals.
A transmitter 100 receives a plurality of information signals
s.sub.1(t), s.sub.2(t), and s.sub.3(t), which are split by a
plurality of splitters 210A, 210B, and 210C. The split signals are
coded by a modulator 104 that acts upon a plurality of carrier
signals produced by a carrier-signal generator 102. Carrier signals
that are coded and modulated with the information signal are
coupled into a communication channel 99 by a plurality of couplers
150A, 150B, and 150C.
At least one receiver 200 receives the coded and modulated carrier
signals. At least one coupler 151 couples the carriers out of the
channel 99 to a carrier separator 221 that separates the received
carrier signals. In this case, the carriers are defined by their
wavelength (or frequency). The carrier separator 221 may be a
wavelength demultiplexer (not shown). The separated carriers are
input to a weight compensator 222 that applies inverse coded
signals with respect to the codes applied to the carriers by the
modulator 104. The weight compensator 222 may compensate for
variations of the code values resulting from distortion in the
channel 99, the coupler(s) 150 and 151, the transmitter 100, and
the receiver 200.
A plurality of carrier signals having different wavelengths are
combined in each of a plurality summing devices 255A, 255B, and
255C. The summed signals are time-domain representations of the
transmitted information signals s.sub.1(t), s.sub.2(t), and
s.sub.3(t). The summing devices 255A, 255B, and 255C may include
signal processors to shape the summed signals or filter the
resulting sums to remove interference and/or noise. Signals output
from each summing device 255A, 255B, and 255C may include one
information signal. The outputs of the summing devices 255A, 255B,
and 255C may be coupled to a multi-user detector (not shown) for
removing interference in signals output from the summing devices
255A, 255B, and 255C.
One of the benefits of the receiver 200 shown in FIG. 24 is that it
achieves separation of signals that interfere in at least one
diversity dimension by processing the signals in a different
diversity dimension. In this case, redundantly modulated carrier
frequencies are combined and processed in the time domain to
demultiplex multiple information signals modulated on the carriers.
Separation of the information signals can be accomplished using a
single-stage weight-and-sum processor and filters instead of a
multi-stage cancellation network.
FIG. 25A illustrates a method of communication that uses
redundantly modulated multicarrier signals. A coding step 300 is an
optional step that may be used to apply a code to at least one
information signal s.sub.n(t). The code may be a spread-spectrum
code, an encryption code, or a multicarrier code that can be used
to help separate multiple received signals. The coding step 300 may
involve applying the code directly to a plurality of carriers
before or after a modulation step 301 in which the carriers are
redundantly modulated with the information signal(s). The carriers
are defined by at least one diversity parameter. The modulated
carriers are coupled into a communication channel in a transmit
step 302. The carriers may be coupled into the channel by more than
one coupler. The communication channel may be a wireless or guided
wave channel. The transmit step 302 depends on the spectrum of the
electromagnetic carriers and the type of channel. The transmit step
302 may also depend on characteristics of the carriers (such as
phase and polarization). The channel may perform or enhance the
coding step 300 as the carrier signals propagate through it. Thus,
the coding step 300 may be affected by the propagation
characteristics of the channel.
A receive step 303 describes the process of coupling the modulated
carriers out of the channel. The method of receiving the carriers
depends on the channel and the characteristics of the modulated
carriers. For example, polarized carriers may be received by
receivers having at least one predetermined polarization. The
receive step 303 may involve coupling signals out of the channel
from multiple couplers. The couplers may be spatially separated or
otherwise separated with respect to at least one diversity
parameter.
The received carriers are separated in a carrier-separation step
304 that separates the carrier signals with respect to at least one
diversity parameter. Separation of the received carriers may be
performed by at least one demultiplexer, such as a wavelength
demultiplexer, a bank of frequency filters, a polarization device,
a spread spectrum decoder, or a time-domain sampler. Each of the
carriers is down-converted to a predetermined frequency band in a
down-convert step 305. Down-conversion may be a heterodyne or
homodyne process. The down-conversion step 305 may involve the
removal of the carrier signal(s) from the information signal(s).
The predetermined frequency band may be the information baseband or
an intermediate-frequency signal. The down-converted signals are
combined in an information-signal separation step 306. The
separation step 306 may involve at least one cancellation method
(such as a weight-and-sum cancellation), at least one constellation
method, or a combination of cancellation and constellation methods.
The separation step 306 may involve a method of nonlinear
processing as part of a method combining cancellation and
constellation processing.
FIG. 25B illustrates a method of communication that uses
redundantly modulated carrier signals. The communication method of
FIG. 25B has the same coding step 300, modulation step 301,
transmit step 302, receive step 303, and carrier-separation step
304 shown in FIG. 25A. The carrier-separation step 304 is an
optional step that may be performed in conjunction with the receive
step 303, or it may be performed as part of an optional decoding
step 307. The decoding step 307 involves compensating for the
variation in the relative phase between at least some of the
received carrier signals due to deliberate coding (such as the
coding step 300) or unintentional coding (such as distortion
resulting from propagation in the channel). The carriers are acted
on by a weight-and-sum step 308 for combining the carriers to
produce a time-domain signal that is processed by a time-domain
processor. Weights applied to the carriers may be complex weights
that adjust phase or add delay to the carriers. The weighting of
the signals may be performed by the decoding step 307. The
weight-and-sum step 308 may involve only summing.
FIG. 25C illustrates a method of receiving communication signals
that include redundantly modulated carrier signals. Redundantly
modulated carrier signals are coupled out of a communication
channel in a receive step 303. The receive step 303 provides
reception via multiple couplers to generate a plurality of
different samples of received multicarrier signals. A carrier
separation step 304 separates the carrier signals from each of the
plurality of couplers. This step 304 may include down converting
the modulated carriers to a common frequency or separating the
modulation signals from the carrier signals. A weight-and-sum step
308 involves coupling different carrier signals from a plurality of
the couplers into a canceller (such as a weight-and-sum canceller)
or a constellation processor. An optional multi-user detection step
309 involves receiving signals output from the canceller and
optimizing detection using an optimization technique. The
multi-user detection step 309 may be incorporated into the
weight-and-sum step 308.
An optimization technique involves any kind of dynamic process for
adjusting weights in an interference-cancellation system/process or
other channel-inversion system/process. Optimization may involve an
iterative update process that causes convergence of the weight
values. An optimization process may include the use of an update
processor that computes a rate of change for each time-varying
weight or channel parameter. The update processor may adjust
weights of a canceller or channel inverter (such as an inverse
filter) in response to a rate of change associated with the
channel. The optimization technique may involve any optimal control
technique including maximum or minimum functions, finite-element
optimizations, and calculus of variations.
FIG. 26 shows a communication system that uses redundantly
modulated carriers to enhance diversity and uses spatial
interferometry multiplexing to increase capacity. Spatial
interferometry multiplexing may provide reuse of the diversity
parameter (e.g., frequency or polarization) that defines the
carriers. A modulator 104 modulates a plurality of information
signals s.sub.n(t) onto a plurality of carrier signals generated by
a multicarrier-signal generator 102. In this case, the signal
generator 102 produces carrier signals having different frequencies
f.sub.1, f.sub.2, and f.sub.3. The modulated multicarrier signals
are coupled into a communication channel 99 by a plurality of
couplers 150A and 150B. In this case the communication channel 99
is a wireless-communication environment. Each coupler 150A and 150B
includes an antenna 158A and 158B, respectively. The antennas 158A
and 158B may be separated antenna elements or antenna-array
processors (not shown).
Modulated carriers may experience spatial gain variations
(spatially dependent variations of their complex amplitudes) due to
propagation effects (such as multipath, shadowing, path loss,
absorption, and scattering) or transmitter 100 parameters (such as
beam shape, carrier weights, information-signal weights, and
scanning). Modulated carrier signals are coupled out of the channel
by a plurality of couplers, such as antennas 158C and 158D. A
receiver 200 separates and processes the information signals
s.sub.n(t) modulated on the carriers. The receiver 200 may include
a multi-user detector and/or a diversity combiner. The receiver 200
may have a design similar to the receiver design shown in FIG. 22A,
FIG. 22B, and/or FIG. 23. The receiver 200 may include a receiver
module (as shown in FIG. 24) for each receiver coupler 158C and
158D.
One benefit of the communication system shown in FIG. 26 is that
the use of redundantly modulated frequency-diverse carriers reduces
the influence of the communication channel on the spatial gain of
the received signals. For example, in a narrow band system (or OFDM
system where different frequency bands carry different information
streams) rapid variations occur in the received signals' gain due
to changes in the signal paths between the transmitter 100 and the
receiver 200. Changes in the signal paths result from relative
motion between the transmitter 100 and the receiver 200. Objects
(such as reflector 160) that move in the communication environment
can cause signal-path changes if these objects reflect signals that
propagate between the transmitter 100 and the receiver 200. The
variation in intensity occurs rapidly (especially at high
frequencies) because path variations as small as a fraction of a
wavelength can significantly effect the gain of the received
signals.
In spatial interferometry multiplexing, weights in a spatial
demultiplexer are set according to training sequences. Transmitted
signals having predetermined values are received and used to
calibrate the spatial demultiplexer. In a flat-fading environment,
the spatial demultiplexer needs to be calibrated frequently.
Frequency diversity mitigates flat fading. Information signals
s.sub.n(t) transmitted on different carriers are combined in the
receiver 200 to generate a plurality of composite information
signals s'.sub.n(t). Because frequency-selective fading has a
minimal impact on the gain of the composite information signals
s'.sub.n(t), large-scale fading effects (such as shadowing and path
loss) may be relied upon to provide the composite information
signals s'.sub.n(t) with predetermined spatial gains. For example,
reflector 160 may provide a large-scale slowly varying effect, such
as shadowing. The reflector 160 blocks the direct path of a
transmission from the transmit antenna 150B to the receive antenna
158D.
Large-scale fading effects require less-frequent updates of the
weights in the receiver 200 than small-scale flat fading. Frequency
diversity can reduce the effects of the channel 99 on
transmitter-controlled and receiver-controlled spatial gain
distributions of the signals s'.sub.n(t). The spatial gain
distributions may be controlled by either or both the transmitter
100 and the receiver 200 using relative positions of couplers,
coupler directionality, masking, polarization, or various
combinations of transmitter and/or receiver control methods.
Although only two transmitter couplers 150A and 150B and two
receiver couplers 158C and 158D are shown, the number of either set
of couplers may be greater. The transmitter 100 may include a
plurality of couplers 150A and 150B as shown in FIG. 26 or the
there may be a plurality of transmitters having one or more
couplers (not shown). The number of receiver couplers (such as
couplers 158C and 158D) may exceed the number of carriers. Likewise
the number of transmitter couplers (such as couplers 150A and 150B)
may exceed the number of carriers. Spatial interferometry
multiplexing may be performed to separate the received signals or
to improve signal quality if both the number of carriers and the
multiple access scheme are sufficient to provide estimates of the
unknown signals. Diversity combining may also be used to enhance
signal quality, preferably when the diversity parameters are not
needed for enhancing system capacity.
The number of received signals that can be separated can be
proportional to the number of receiver couplers. The number of
received signals that can be separated is also related to the
number of carriers and the techniques used to detect and separate
signals. For example, in time-domain processing, the signals may
overlap each other. A simple multi-user detector (included in the
receiver 200) may separate the overlapping signals to provide a
substantial increase in bandwidth efficiency. Similarly, spectral
overlap of orthogonal carriers improves the spectral efficiency of
the communication protocol. Spatial interferometry multiplexing is
a type of multi-user detection that separates signals received by
spatially diverse, angle-diverse, or polarization-diverse receivers
by canceling interference from the desired signals. Combining
spatial interferometry multiplexing and multi-user detection based
on a different diversity parameter can enhance capacity, enhance
diversity, or enhance both capacity and diversity benefits.
FIG. 27 shows a receiver 200 that receives multicarrier signals and
achieves benefits of spatial diversity, frequency diversity, and
capacity enhancement of spatial interferometry multiplexing. The
receiver 200 has a plurality of couplers 150A, 150B, and 150C
coupled to a communication channel (not shown) to generate a
plurality of samples of multicarrier signals. The communication
channel (not shown) may be a waveguide or wireless channel. The
multicarrier signals received at each coupler 150A, 150B, and 150C
are separated by a diversity-parameter demultiplexer 210A, 210B,
and 210C.
In a WDM system, the demultiplexers 210A, 210B, and 210C are
wavelength demultiplexers. In a wireless multicarrier system where
each carrier is defined by its frequency, the demultiplexers 210A,
210B, and 210C are filter banks or frequency-separation processes
that spectrally decompose the received signals into a set of
frequency bins. A frequency bin represents the frequency band of a
filter in the filter bank. The demultiplexers 210A, 210B, and 210C
may include down converters, such as heterodyne or homodyne systems
(not shown) to recover modulated information signals from each
carrier or to convert each carrier to a common carrier signal. A
common carrier signal may be defined by an intermediate frequency.
The demultiplexers 210A, 210B, and 210C may provide orthogonal
outputs (such as separate carrier frequencies) or non-orthogonal
outputs in which at least one separated carrier is not entirely
separated from at least one other carrier.
Each of the separated carriers is coupled into at least one of a
plurality of weight-and-sum systems 255A, 255B, and 255C. In this
case, each separated carrier from each demultiplexer 210A, 210B,
and 210C is coupled into different weight-and-sum systems 255A,
255B, and 255C. If the number of weight-and-sum systems 255A, 255B,
and 255C exceed the number of carriers, then multiple carriers from
at least one of the demultiplexers 210A, 210B, and 210C may be
coupled into at least one weight-and-sum system 255A, 255B, and
255C. The weights applied by the weight-and-sum systems 255A, 255B,
and 255C may be deterministic or adaptive. Delay or phase-alignment
units (not shown) may be incorporated into the weight-and-sum
systems 255A, 255B, and 255C. Signal outputs from each of the
weight-and-sum systems 255A, 255B, and 255C include at least one
substantially isolated information signal s.sub.n(t). The
weight-and-sum systems 255A, 255B, and 255C may include filters
(not shown) or digital signal processing systems (not shown) to
enhance reception of desired signals and mitigate interference and
noise. The signal outputs from the weight-and-sum systems 255A,
255B, and 255C may optionally be coupled into a multi-user detector
256.
The receiver shown in FIG. 27 may be adapted to include a larger
number of couplers (such as couplers 150A, 150B, and 150C) than
carrier signals. The number of transmitter couplers and the number
of receiver couplers may each exceed the number of carrier signals
transmitted or received by each coupler. The couplers may be
spatially separated antennas, array processors, or optical
couplers. Although the couplers 150A, 150B, and 150C shown in FIG.
27 are spatially separated, the couplers 150A, 150B, and 150C may
have any type of diversity, such as spatial, directionality,
polarization, path, phase-space, time, or code diversity. The
couplers 150A, 150B, and 150C may be diverse in more than one
diversity parameter. Similarly, the carriers may have any
combination of diversity parameters.
Carrier signals shown in FIG. 26 and FIG. 27 are represented as
multi-frequency (or multi-wavelength) signals. These carriers (as
well as carriers represented in other figures) may be coded. For
example, a group of direct-sequence CDMA codes may be created from
the appropriate selection of weights applied to each of the
carriers. Although coded, the carriers are still redundantly
modulated with information signals. The weight-and-sum systems
255A, 255B, and 255C shown in FIG. 27 may include one or more
correlators (not shown) and/or one or more matched filters (not
shown) for acting on either or both the time-domain and the
frequency-domain signals resulting from the carriers. The receiver
200 in either FIG. 26 or FIG. 27 may perform multi-user detection
between either or both the time-domain and frequency-domain coded
carrier signals.
Linear cancellation processes (e.g., weight-and-sum processes)
require a number of algebraically unique equations that equals or
exceeds the number of unknown values. FIG. 28A shows a receiver 200
that receives a deficient number of receive signals 265 (i.e.,
equations) and applies a nonlinear process 266 to at least one of
the equations to generate one or more additional algebraically
unique equations. The nonlinear equation(s) have more than one
solution. Therefore, some information about the unknowns (such as
possible values) is required to explicitly solve the equations. A
multi-user detector 256 processes the equations. This processing is
supported by information about the unknowns 267, which is input to
the multi-user detector 256. Information about the unknowns may be
acquired through training or estimation processes.
FIG. 28B shows steps of a method for separating unknown signals in
a plurality of received signals. A number M of equations having N
unknown signals is received in a first step 270. A determination of
M.gtoreq.N is made in a second step 271. If the result is "yes,"
the equations may be processed by cancellation and/or constellation
methods 272. If the result is "no," the equations may be processed
by a combined cancellation and constellation process 273, a
constellation process 275, or a cancellation and/or constellation
method that uses a nonlinear process 274. The non-linear process
274 may be applied to at least one of the equations to provide an
additional number of equations. The equations are passed on to a
cancellation step 280 that uses information about at least one of
the unknowns (such as possible values) to explicitly solve the
equations.
FIG. 29 illustrates a method of solving M linear equations having N
unknown signals where M<N. M received signals are obtained from
a sampling step 270. A sampler (not shown) is used to provide a
plurality of samples (received signals) that have linear
combinations of at least one of the N unknown signals. The number M
of samples is compared to the number N of unknowns in a comparison
step 271. If M<N, a nonlinear processing step 274 acts on at
least one of the samples. An output step 286 provides one or more
additional algebraically unique equations (which are nonlinear
equations) such that M.gtoreq.N. An equation-solving step 287
provides solutions to the M equations. However, at least one
solution contains more than one set of possible values. An
information step 288 involves submitting additional information
about the unknown signals (such as possible values for the unknowns
or relationships between the values of the unknowns) so that the
equations can be solved explicitly (or at least used to provide
estimates having a high degree of accuracy) in a decision step 289.
The values of the unknowns are output in an output step 290.
Many of the interferometry-multiplexing protocols (which use
interference cancellation and other multi-user detection schemes)
achieve increased bandwidth efficiency by indirectly exploiting the
dimension of power. For example, differential modulation schemes
require greater power levels to enhance capacity while maintaining
the same BER or SNR as simple modulation. For example, the SNR of
an M-ary amplitude modulation (AM) scheme (such as quadrature AM)
depends on the difference between the AM steps. BER or SNR in a
interferometry-multiplexing protocol depends on the difference
between signal levels of the desired signal and the interference
(which may be other desired signals). Also, the additional antennas
used in spatial interferometry multiplexing cause increased noise
levels.
M-ary AM requires an approximately doubling of the required system
power for each increment in the number M. The system power ranges
from proportions of 0 to 2.sup.M-1 depending on the transmitted
data symbols. A multiple-access version of M-ary AM is
differential-ASK modulation. The most basic implementation of
differential ASK involves each of a plurality N of transceivers
(such as transceivers 121A to 121G shown in FIG. 30) transmitting N
different information signals that each have a binary set of values
received at one or more transceivers (such as a base transceiver
120). The "on" value of each n.sup.th received signal corresponds
to a unique signal level, which may be proportional to 2.sup.n
where n=0, . . . (M-1). The "off" value of each transmitted signal
is zero. In order to optimize power conservation, nearby
transceivers 121A, 121B, and 121C may generate the highest-power
received transmissions at the base transceiver 120.
Differential-modulation techniques may be used in array systems
where multipath fading, shadowing, and other channel effects can
assist in the differential qualities of the received signals to
optimize power conservation. A more complex version of the
differential-ASK protocol uses more than two signal levels for each
transceiver.
FIG. 31A and FIG. 31B show SNR variations of signals separated from
a 3-element array and a 4-element array, respectively. The y-axis
shows SNR and the x-axis indicates average interference as a
fraction of the desired signal level received by an array element.
The received power level for each desired signal is set to the
average power corresponding to the differential ASK that supports
the same number of users (channels). A SNR of 30 dB is an arbitrary
baseline set for differential-ASK performance. A SNR of 30 dB or
higher is achieved by a 3-element array for average interference
levels below 0.42. A 4-element array exceeds differential-ASK SNR
for average interference levels below 0.48. Although there are
conditions in which an interferometry technique (such as spatial
interferometry multiplexing) provides superior power efficiency
and/or signal quality compared to differential-modulation
techniques, differential modulation (such as differential ASK) does
not require multiple antennas or cancellation systems.
FIG. 32 shows a two-dimensional signal space comprised of a spatial
dimension and a differential-power dimension. The signal space can
be designed to achieve an optimal compromise between several
factors including system complexity, power efficiency, BER or SNR,
and stability. Diversity parameters may be used to enhance signal
quality. For example, effects of the multipath-fading channel can
be mitigated by providing time-domain signals with frequency
diversity. The signal space can be implemented to optimize
operating characteristics, such as, but not limited to power
efficiency, BER, SNR, signal to noise plus interference, and signal
strength.
In waveguide and/or wireless communications, a remote transceiver
may communicate a desire to initiate communications to a central
unit or another transceiver. The central unit or other transceiver
may respond with information indicating phase shifts or delays to
be applied to the transmitted signals to synchronize the
transmissions with respect to other transmissions that the
transceiver is receiving.
A similar type of feedback may be used to optimize either or both
transmitter and receiver parameters. FIG. 33 shows a process for
calibrating weights in a multi-user detector (not shown), such as a
canceller or constellation processor. A first step 310 involves
determining the values of received signals during normal
transmissions. This is performed by a cancellation and/or
constellation method. A second step 311 involves making a decision
on whether to initiate recalibration of the processor weights. The
decision step 311 is made based on analysis of at least one
parameter, such as BER or other diagnostics, established
benchmarks, process-control signals, or timers. The decision step
may be made based on either internal or external parameters. If one
or more predefined conditions are met, an initiation step 312
starts a recalibration sequence. The initiation step 312 may convey
a message to one or more transmitters to initiate calibration, or
training sequences may be initiated at predetermined intervals, in
which case, the initiation step 312 only involves the receiver
preparing itself for calibration.
Known training signals are received in a receiving step 313, which
is followed by an optimization step 314. In the optimization step
314, an optimization technique (defined previously) is used to
adjust the weights based on one or more criteria that are balanced
or optimized.
A looped optimization process is shown in FIG. 34. Values of
received signals are determined in a first step 310. A decision
step 315 may involve making a decision on which parameter to adjust
on either or both the receiver and a remote transmitter, and how to
adjust the parameter (e.g., magnitude and/or direction). The
decision step 315 may make a decision to break out of the loop and
return to a data transmission/reception mode. The decision step 315
is followed by a parameter-adjustment step 316 in which one or more
parameters are adjusted, or control signals are generated that
result in parameter adjustment. Control may be returned to the
first step 310.
Training sequencers may be used in transmitters to transmit
training sequences consisting of predetermined signals. The
training sequences are processed by receivers (as described in FIG.
33 and FIG. 34) to adjust weights applied to received signals.
Weights are applied to optimize the desired signal. The
optimization process may be performed with respect to
signal-to-noise, signal-to-interference,
signal-to-noise-plus-interference, bit-error rate, or any type of
measurement or computation that indicates signal quality. A
training sequence may be transmitted in a parallel channel, or the
training sequence may be part of a received signal. For example,
amplitude measurements of a constant-modulus signal may be regarded
as a training sequence. The optimization process may be performed
by either or both transmitters and receivers. In open-loop
processing, a transceiver analyzes received signals and adjusts its
transmissions to compensate for the channel. A closed-loop process
involves feedback between a receiver and a transmitter. The
closed-loop approach is more robust to changes in the propagation
medium.
FIG. 35 illustrates a method of adjusting reception parameters and
assigning transmissions to signal spaces in order to optimize
system-operating parameters. Signals are received in a first step
295. The signals may be information or training signals. The
spatial gain(s) of the received signals are determined in a
measurement step 296. Signal parameters are optimized in an
optimization step 297. The optimization step 297 includes
optimizing signal parameters (such as SNR, signal power, and BER).
A second optimization step 298 includes distributing signals into
optimized signal spaces. The optimization is based on system
parameters (such as power efficiency and system complexity).
Although the signal space shown in FIG. 32 has only two dimensions,
signal spaces may have three or more dimensions. Signal-space axes
may include any diversity parameter or combination, including
differential-modulation dimensions. Although differential ASK is
shown, any type of differential modulation scheme may be used
including hybrid schemes that combine different types of modulation
schemes in at least one form of differential modulation.
Multiple forms of diversity may be included in the communication
systems described in this specification. These forms of diversity
may be used for either or both signal enhancement and capacity
increases. Each diversity parameter may be dedicated to one
particular use of diversity, or the diversity parameters may be
adjusted to provide an optimal combination of both
signal-enhancement benefits and capacity improvements. In
cancellation and constellation systems, the number of samples
representing linear and nonlinear combinations of unknown signals
is increased by providing additional diversity dimensions. One
example shown in the specification describes an addition of
spatially separated frequency-diversity cancellation systems.
However, many other types of diversity-parameter combining may be
used to increase the number of algebraically unique combinations of
unknown signals or to improve signal quality.
A benefit of generating more samples of unknown signals in a
cancellation system includes enabling the values of the unknowns to
be determined explicitly. In a constellation system, generating
more samples improves the BER of the decision processes. A benefit
of constellation methods is that they can be used to obtain signal
estimates without the hardware and processing requirements of
cancellation methods. A constellation system is scalable to
increased numbers of unknowns. Under high demand, a constellation
system (like quasi-orthogonal CDMA) enables a graceful degradation
of signal quality resulting from increased higher BER.
Constellation processing may also be performed in addition to
cancellation processing to help determine processing accuracy and
to indicate when recalibration is necessary.
In the preferred embodiments, several kinds of interferometry
multiplexing are demonstrated to provide a basic understanding of
diversity reception and spatial demultiplexing. With respect to
this understanding, many aspects of this invention may vary. For
example, signal spaces and diversity parameters may include
redundantly modulated signal spaces. The antenna arrays may be
arrays of individual antennas, a lens system, or a multiple-feed
single-dish antenna where each feed is considered to be an
individual antenna element. Although only two- and three-element
cancellers are shown, cancellation processes may be performed on a
larger number of inputs. The complexity of the cancellation process
typically increases for larger numbers of inputs. A CPU may be used
to perform the weight-and-sum operations or equivalent types of
cancellation processes that result in separation of the signals.
Although the wireless interface in the invention is described with
regard to RF and microwave frequencies, the principles of operation
of the invention apply to any frequency in the electromagnetic
spectrum. Additionally, a demultiplexer may include combinations of
space, frequency, time, phase space, mode, code, and
polarization-diversity combining methods. Furthermore,
constant-modulus signals may be transmitted in the communication
system. Constant-modulus transmissions can simplify the
demultiplexing of received signals. In this regard, it should be
understood that such variations as well as other variations fall
within the scope of the present invention, its essence lying more
fundamentally with the design realizations and discoveries achieved
than merely the particular designs developed.
This invention claims the methods of controlling signal parameters
in multiple diversity dimensions to achieve specific signal
processing capabilities (such as diversity benefits and capacity
enhancement) in other diversity dimensions. For example, PCT Pat.
Appl. No. WO99/41871 describes how different-frequency carriers
transmitted from different spatial locations cause a time-varying
superposition beam pattern (and a time-varying spatial gain
distribution). This enables time-domain processing to yield either
or both diversity and capacity benefits.
The foregoing discussion and the claims that follow describe the
preferred embodiments of the present invention. With respect to the
claims, it should be understood that changes could be made without
departing from the essence of the invention. To the extent such
changes embody the essence of the present invention, each naturally
falls within the breadth of protection encompassed by this patent.
This is particularly true for the present invention because its
basic concepts and understandings are fundamental in nature and can
be broadly applied.
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